Turbo trainer is about the best way but its soooooooooooooooo boring.

You can pick them up for about 70 new and sell them on for a decent price or buy 2nd hand in the first place, theres always someone injured after one so they're easy enough to sell again.

I got dragged out doing loads of walking in the same situation with arms and wrists and that deffinately helped keep my fitness but again its not exactly interesting, it does get you out though i guess and i found plenty of new stuff to ride.

I can't really ride at the moment, or do any kind of intensive exercise (so kickboxing and running are out too) and I am really enjoying walking - I find it very relaxing to have to go slowly.  It's different if you aren't going anywhere, nice and relaxing.  And it's better than sitting on my ass.

I would imagine that at the moment, anything involving impact (like running) would hurt like hell?

Swimming is always a good one for rehab when you are allowed.  And if the roads are OK round you, going out on a road bike - far less dull than a turbo (personally I would rather die than use a turbo - I tried when I had 9 months off with a broken shoulder, but I only used it twice, and binned it)