I started out squats by doing them in a doorway in my house. I could hold onto the doorjamb for balance and to help me get back up (because I COULD NOT do even one when I started.) That helped a lot.

I still do plank on my stairs. I can't do a prone one, yet. But stairs means you can gradually work your way lower, over time as you get stronger.

I also use doorways for lat pulls. I grab the doorjamb and lean back, then walk my feet forward until I am inclined as far as I can be safely, without either hands or feet slipping. It's NOT perfect, but it's something.

Also, shovelglove is ridiculous fun weight and flexibility exercise. *Be careful*, though, of your back in the twisting moves. http://www.shovelglove.com/