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.