Ukraine sould just confiscate any gas it needs from the Gasprom pipelines, say that it is in leiu of “rent” for letting Russia use Crimea and then call it “even”.
Either that or Ukraine should simply say to Russia if you want Crimea that's fine but you have to pay for it. Forgiveness of all current Ukraine debt, low energy prices for a given time and a amount of hard cash to save face.
It seems that the vast majority of the people living there want to be Russian so keeping them in Ukraine will always be problematic.
If they stay on their current path they will end up just like Georgia with nothing but a protracted crisis and nothing to show for it.
Hey we bought Alaska. Just thinking outside the box.