What are all the ways a player can get to the Astral Plane?

Question

I am planning a campaign where the characters must travel to the astral plane several times, though each subsequent time they lose one of the means to do so (i.e., Astral Projection spell, through the Ethereal plane, etc).

By "lose one of the means to do so", this counts portals generated by the same method as one method; using Astral Projection once (no matter who cast it) as one method and, hence, that option would not be available to them (and only just to them); a "Bag of Holding thrown into a Portable Hole or Handy Haversack" is one method, regardless of the 3 different combinations of these items that would generate this effect; a teleportation circle, regardless of which one, counts as one method. etc.

For planning purposes, I want to scour the PHB, DMG, and MM to determine all the possible ways the players can achieve this and drop subtle hints of such as they progress.

To assist in the effort, please answer: What are all the ways that exist for a PC to enter the Astral Plane? No restrictions or caveats need apply. For example, a way that would get a PC to the Astral Plane but kill them in the process counts.

Answer 1

Ways to get to the Astral Plane specifically:

• Wild Magic Sorcerer's Wild Magic Surge
• Astral Projection spell
• Robe of Stars item
• Placing an item that creates an extradimensional space inside another extradimensional space

Ways to get to other planes in general:

• Plane Shift spell
• Gate spell
• The violet layer of a Prismatic Wall spell
• Amulet of the Planes item
• Cubic Gate item
• Well of Many Worlds item

Ways of doing just about anything:

• Level 10+ Cleric's Divine Intervention
• Wish spell

Answer 2

Miniman's answer addresses the varied player-facing methods present in the game. This answer will thus constrain itself to DM-facing methods (i.e. methods that are the purview of cosmological interactions or the abilities of various creatures which the players might seek out).

Monster-related methods:

• Seek out the aid of a zerth, an advanced Githzerai monk. These creatures can use their potent psychic abilities to replicate the effects of a Plane Shift spell, only it's psionics instead of magic. Githyanki knights can also do this, but they are a lot less likely to knowingly help your players.
• Hijack a Githyanki Spaceship. The Githyanki entry mentions that they raid worlds "from the decks of their astral vessels" (in addition to the backs of Red Dragons). While no further details are provided, such vessels would surely be capable of travelling from the Githyanki strongholds in the deep Astral to the Material Plane.
• Enlist the aid of a Sphinx. A Sphynx can provide transport to other planes of existence, and Gynosphinxes are known to do so in order to test supplicants. Furthermore, Gynosphinxes are called out as open to bargaining for their services.
• Find a Demonic Portal. "Wherever they wander across the Abyss, demons search for portals to other planes". Some of those portals may well lead to the Astral, though finding such a portal will likely require successfully entreating the aid of the Abyss itself, which is known to reward extraplanar outsiders who manage to usurp control of one of its layers from a Demon Lord.
• Primus, Lord of the Modrons, seems to be able to move things across planes, having placed the Spawning Stone in Limbo and periodically sending modrons all across the planes. Whatever method Primus employs can probably see the characters to the Astral, if the Modrons can be convinced to use it on them.

Cosmology-related methods

• Get summoned. Many creatures are capable of being summoned across planes through appropriate rituals. Most of the time, a creature's True Name and some material components seem sufficient. While most player characters are not of a race typically summonable via True Naming, such characters can nonetheless utilize this method, either by gaining a True Name or summoning ritual in some manner, or by possessing a creature that itself has a true name.
• Walk. The Ethereal Plane in the Great Wheel cosmology lets you just walk to most other places, by traversing the Deep Ethereal. Furthermore, Ether Cyclones, should they still exist, often deposit travelers in the Astral. The Shadowfell, which used to be the demi-plane of Shadow, may have similar properties, and may (now that it isn't just another part of the Ethereal) count as a separate method.
• Find a permanent, rather than Abyssal, portal. Abyssal portals (those mentioned above by reference to demons) are also referred to as Abyssal breaches and rifts and, given their changing locations, are likely not the same sort of travel as the more traditional and classic permanent gateways.
• Find the other side of a Color Pool and climb through it. While Color Pools are visible from only one of their Astral sides, travel through them from the other direction is not specifically impossible. Probably more than just walking will be required to make it through, though.
• Be born on the Astral. If the Astral is your Native Plane, you can get sent back there via a whole host of other methods, like Banishment.
• Die on the Astral. If you die on the Astral, you can get back to it by being raised from the dead there. Bonus points if you turn into a ghost first, so that you can manifest on the Astral from that plane's border Ethereal.
• Worship a deity whose domain is (on) the Astral and later die. There aren't any such deities mentioned so far in published material (though Gith may be a good candidate for worshippers), but there might some (or the players might be able to trigger the apotheosis of one) and such a deity's worshippers would be transported to the Astral upon death.