Sorry to hear that you've had injuries. That seems to happen to me a lot. I get gung-ho about something and then wind up injured.
I have cervical spine issues and was told to avoid all high impact exercises and lifting of heavy weights, especially overhead.
I do low impact walking dvds by Leslie Sansone, or ride my stationary bike, or just march in place/do my own thing/dance or whatever while I watch tv.
For keeping up my muscles, I do light weights (no more than 5 lbs) for the typical exercises: bicep curls, tricep kickbacks, front shoulder raises, rows (I use a theraband for that, that I got at physical therapy). Also, squats, lunges, and calf raises for the legs. I do an occassional yoga dvd.
The main thing is to not get over-zealous and injure yourself again.
I've come to realize that I want to grow old gracefully and pain free.
Best wishes