Identifying Character Abilities
A 6th level Druid has access to the Druidic language, three cantrips, two uses of Wild Shape for 1 hour each, four 1st-level spell slots, three 2nd-level and three 3rd-level spell slots. The Circle of the Shephard nets you Speech of the Woods, Spirit Totem, and Mighty Summoner. The Tortle race grants you Shell Defense and proficiency with Survival.
Identifying Risks of Camping in Hostile Territory
In order to know what the optimal precautions are, we have to identify the risks, which I will define as anything that could inflict damage, disease, or a condition to one of the players, or interrupt the long rest:
- Extreme Cold or Extreme Heat (DMG page 110)
- Frigid Water (DMG page 110)
- Quicksand (DMG page 110)
- Razorvine (DMG page 110)
- Thin Ice (DMG page 111)
- Melee attacks (primarily focused on beasts)
- Ranged attacks (primarily focused on sentient beings)
- Spells from hostile creatures
- Catching Sewer Plague from a rat (DMG page 257)
- Catching Sight Rot from tainted water (DMG page 257)
- Falling trees ("widowmakers")
Mundane Risk Mitigation
Right off the bat, a Wisdom (Survival) check would be appropriate to find the best campsite nearby, preferably a location with good drainage to mitigate the effects of flooding, Frigid Water, Quicksand, and Thin Ice. Digging a small trench around the campsite can guide water runoff in case you are unable to get off the ground, which may mitigate some of those risks to a lesser extent.
It also would likely not be of much risk for the Druid to spend some time leaving a few hidden messages in Druidic a short ways away from the camp, letting other Druids know of your presence and that you come in peace. Alternatively, you could leave threats for them not to engage, and have the message describe an area a ways away from your camp. You could then set up some traps in this second camp to hinder them and alert you of an attempted attack.
A fire can stave off Extreme Cold, and can also be used to boil water which may prevent Sight Rot. However, a fire can also draw the attention of hostile creatures. Therefore, the ideal setup would be to obscure the light from the campfire from outside the camp.
Using Artisan Tools to Mitigate Risks
Druids have proficiency with the Herbalism Kit, but with Tasha's optional rules you could have proficiency in any tool. Using the optional Tool Descriptions in Xanathar's Guide to Everything (pages 78-85), the following would be beneficial to mitigating the listed risks:
- Alchemist's Supplies: As part of a long rest, craft one dose of acid, alchemist's fire, antitoxin, or oil. The antitoxin grants advantage on saving throws against poison for one hour (PHB page 151), while the others can be used to deal damage to intruders.
- Alchemist's Supplies: As part of a long rest, purify up to 6 gallons of water. This would mitigate catching any water-borne illnesses, such as Sight Rot.
- Carpenter's Tools: As part of a long rest, you can construct a lean-to or a similar shelter to keep your group dray and in the shade for the duration of the rest. The shade may prevent the effects of Extreme Heat. Placing the campfire under the shelter might prevent Strong Winds (DMG 11o) or Heavy Precipitation (DMG 110) from extinguishing the fire, which might otherwise prevent the players from mitigating the risks due to Extreme Cold.
- Herbalism Kit: You can identify most plants with a quick inspection of their appearance and smell. This can mitigate Razorvine, or other plants that might cause the poisoned condition.
- Thieves' Tools: As part of a short rest (which is up the the DM to allow this as part of a long rest instead), you can create a trap using items you have on hand. The trap deals damage appropriate to the materials used in crafting it (this is especially effective if you also have proficiency in Alchemist's Supplies). Sample traps can be found on pages 122 - 123 of the DMG. The Falling Net and Pits should be able to be set up in any environment. Additionally, Xanthar's has the Crossbow Trap on page 114. Other traps may involve just setting off an alarm, such as a tripwire causing a string of bells to ring.
- Weaver's Tools: You can create an outfit for a creature as part of along rest. This can be used to mitigate the risks of Extreme Cold.
- Giant Badger: with a 10 foot burrow speed, you can loosen the earth around the perimeter of your camp, although the dirt presumably gets filled in behind you as you move. This can be used in tandem with the Move Earth cantrip to create effectively a wall up to 10-foot high around camp. This is inspired by how the Romans set up their camps. This wall would effectively conceal the light from a campfire and also block line of sight from ranged attacks and some spells' line of sight requirement. The wall would also hinder melee combat.
- Mold Earth (cantrip): described above, used in tandem with Wild Shape
- Guidance (cantrip): this would be best used when using your initial Survival check to locate the best campground
- Plant Growth (3rd level): creates permanent difficult terrain in areas with vegetation. Center the spell on the camp but exclude the camp itself to mitigate melee attacks.
- Purify Food and Drink (1st level): mitigates poison and disease
- Detect Poison and Disease (1st): Mitigates poison and disease by warning you about it
- Protection From Poison (2nd): This neutralizes one poison, and mitigates one creature being poisoned for an hour
- Create or Destroy Water (1st): This can mitigate water-borne diseases by bypassing the need to gather potentially tainted water
- Goodberry (1st): as you mentioned, mitigates poison by bypassing needing to forage
- Animal Friendship (1st): this spell can be used in tandem with your Speech of the Woods feature to net you a nocturnal beast ally that, at the very least, can screech or howl or make some other loud noise to wake you if it sees a predator or humanoid coming towards your camp.
- Water Breathing (3rd): Arguably of lesser importance, but if you're in an area where flooding or Thin Ice is a concern, this this would be very useful. In a pinch, you may also be able to mitigate some spells' line of sight requirement by having everyone sleep in pits filled with murky water. This may result in Extreme Cold, however.
- Darkvision (2nd): As you mentioned, being able to see in the dark, especially if you are trying to avoid or just can't light a fire, would be a huge boon for the 2 hours you are able to stay awake.
- Snare (1st): I would use this in tandem with Plant Growth to create choke points. Putting two or three of these adjacent to each other could also mitigate creatures jumping over one of the snares.
- Fog Cloud (1st): Can be used to hide a campfire for an hour, or to mitigate ranged attacks or line of sight for some spells.
- Call Lightning (3rd): This can mitigate stormy weather by letting you control where lighting strikes
- Conjure Animals (3rd): Conjure two Giant Spiders, and have them web up as much area as they can within the hour. You may also be able to find a giant spider to cast Animal Friendship on, in which case the giant spider would be able to sense the exact location of any creature that touches the web. Webs may also be used in tandem with alchemist's fire or a fire-summoning cantrip such as Produce Flame, Create Bonfire, or Druidcraft. Using these webs in tandem with the wall-and-rampart from the Giant Badger and Move Earth combination would give the webs some vertical space instead of just laying flat on the ground, especially if there are no trees. Per Webs (DMG 105):
Giant spiders weave thick, sticky webs across passages and at the bottom of pits to snare prey. These web-filled areas are difficult terrain. Moreover, a creature entering a webbed area for the first time on a turn or starting its turn there must succeed on a DC 12 Dexterity saving throw or become restrained by the webs. ... Each 10-foot cube of giant webs has AC 10, 15 hit points, vulnerability to fire, and immunity to bludgeoning, piercing, and psychic damage.
So, with all of that laid out, I think the most efficient fortifications would be to request to find the most suitable campsite to prompt a Survival check. You should do well enough to mitigate most environmental dangers with that alone. You would then leave a couple of hidden messages in Druidic warning that your campsite is a dangerous place while you are gathering enough firewood for the night. Cast Animal Friendship on an owl (or another creature with darkvision) and tell it to wake you if it sees a predator approaching camp.
Next, Wild Shape into a giant badger to till up the perimeter of your camp, and use Mold Earth to set up a rampart (excavating the dirt towards the center of camp to make the structure effectively be a 10-foot wall of earth). You may want to also use this time to loosen some pockets of earth outside of camp to use as pit traps. Then use a 3rd-level spell slot to cast Conjure Animal, summoning two giant spiders and have them form giant webs all along the outside of the wall and fill in the moat. Optionally, use another 3rd-level spell slot to cast Plant Growth if there is enough vegetation around.
If you have any relevant tool proficiencies and your DM is using the optional tool uses in Xanathar's, craft whatever you can during this long rest to use as a trap (you should be able to disarm any untriggered traps in the morning to reuse the next night). Finally, light a campfire with the knowledge that the light will likely not be seen by creatures on the ground.
While sleeping, you may be able to utilize your Shell Defense to gain +4 AC while you rest. If you are woken up, set the webbing on fire. However, this setup should deter all but the most determined creatures from messing with you at night, considering that from outward appearances, your camp is just a giant spider nest.