If you have some cans of food, you can use them as hand-weights. You can do lite yoga, pilates, and Tai Chi (my favorite) -- you can find free videos on Hulu.com or YouTube for them i.e. Eccentric Arm Yoga -- don't be fooled, it's harder than it looks but a good workout. Look at all her videos; she's awesome and Canadian and very cute ... OK, I'm biased just a bit.
Here's my list for people with challenges (I also have a bum knee) -- chair aerobics (look on the net for this too); leg lifts (side & back), knee lifts, calf lifts, butt squeezes, tummy tucks, thigh presses, do arm press & shoulder presses with cans, and sit-ups (you are already doing).
I was gonna suggest skipping rope but not sure if you can do that or not. I also do mock flags (I actually have mini-Canadian flags for this) -- you know the kind the airport guys do; good for the arms & shoulders (I also do these to bouncy music).
And hey, don't forget dancing (don't laugh, it's good exercise; try western style line dancing, if inclined). Besides, it's good practice for when you find your lady love.
You can also try MOCK Boxing; it's where you pretend to be boxing and hitting the bag, but no equipment. You can also try "Walk off the lbs" videos: Leslie Sansoon is good (you might be able to find some free on YouTube too).
You can create your own exercises and have a whole indoor routine in no time -- you are only limited by your imagination ...
As for MOTIVATION: you pretty well just have to make a commitment to getting healthier; and the best way to do that is by scheduling & planning your goals. I find that helps a lot ... good luck!