Sponge - I don't know if you enjoy swimming - even if you are just hanging onto the ledge and kicking your legs. Even sitting down, raise your legs and lower them. Doing this numerous times a day a couple of raises is better then doing a whole bunch at one time and then getting worn out. Even if you raise you knee and move your foot can help. Remember though, you do not want to make this painful - always check with your doctor to ensure you can do this.
I hear you about getting weaker - I use a walker too, but so far am able to remain mobile. However, I do try to move my legs as much as possible as I find if I remain still it gets worse and stiff. If you can see a physiotherapist - they can help design a workout that allows you to work at your capabilities.
In the meantime, any movement you CAN do, I would encourage this. I like dancing on days I can't get out due to weather - Even when I just hang onto the wall for support and move my body - lift my feet - any movement that feels good will help. Anything I do that hurts I stop.
Just my own opinion.