Single leg bridges:
http://www.tinajuanfitness.info/exer...g%20bridge.htm
These can also be done on a swiss ball! and made quite alot harder as you have to control the rotation of the ball.

Clam Shell:
http://imagine123.com/fitness/tag/clamshell-exercise/

There are 2 simple ones that won't stress your knees too much.

Have fun.