For group enemies, add more monsters
half-again-as-many monsters here is a good rule of thumb, since you have half-again-as-many players, as other answers have already pointed out. You also can vary the sort of monsters you add and occasionally add hp or abilities to an existing monster or monsters to help with the blandness of simply adding more identical creatures, as other answers mention.
For small traps, add more of them elsewhere
Traps like "covered 10' pit with sharpened candy canes, 1d6 falling+1d6 piercing, Wis(perception) DC 20 to notice" can just have an equal number thrown around the module as extras-- after running into them the first time the players will be on the lookout and are less likely to be caught unawares by repeats, but combat or other eventualities might force them into one nonetheless.
For big traps, adjust for the additional players manually
A room with 5 chairs that all need to be sat in to open a door to some treasure, but then the chairs bind the PCs, obviously needs 8 chairs now instead of 5. Similarly, if the metal grating that falls, trapping the PCs in the path of onrushing molasses, weighs enough that 4 strong PCs working together should be just able to lift it, then now it should be heavy enough that 5 or 6 PCs are needed. If Jack Frost asks the players a riddle in exchange for not eating their souls, no adjustment at all is necessary.
For solo monsters, give them legendary and/or lair actions
legendary and lair actions help solo monsters keep up with players in the action economy. While typically only cool, high-level boss monsters get legendary actions, actually all of them should and the system works great. Make up some minor off-turn actions for the solo monsters to take (typically including at least one minor attack against an adjacent opponent, one way of struggling against bad positioning, and one cool "Behold my true power!" ability that takes
all of their actions), reusing parts of their regular actions where applicable, and give 'em 3 legendary actions per round. If they already had legendary actions, give them an extra one each round or add a lair action. I've found legendary actions much more satisfying that lair actions for bosses, but minor solo enemies could probably make better use of lair actions instead, so they don't detract from the awesomeness of the boss.
For noncombat, nontrap challenges, do nothing
Stuff like opportunities to use Herb Lore or an encounter with friendly gingerbread men that provide information on the local area don't need to be adjusted. It's true that this makes the larger party much more likely to get all the things and make all the optional checks and gather all the people-based information, but adding more of these checks takes much too much time (NPC dialogue in a group of 8 is at a premium already, and Herb Lore checks etc only involves characters that have the relevant skills) and making the checks harder means its likely that one of the players will fail at the thing they specialized in the only time that specialty comes up, which is no fun. Leaving it as-is lets people potentially get a little bit of premium spotlight time without spending too much time on individual player activities.