So, let's clear this out. You CAN go back from the Crater, unless you're in the final part.
Whoever you have in your party at that point will tell you that there's no coming back or something similar and ask you if you're sure you want to go on.
If you choose no, you can go back and forth as many times as you want.
If you choose yes, you proceed to finishing the game, because you only have the final battles with Jenova and Sephiroth left and you can't rest or save anymore.

Edit: Of course, to go back, you just have to...errr, go back. Walk to where you were before. You can board the Highwind again and go wherever you like. Also do sidequests if you want to. Actually, most players choose just this moment to do sidequests.