All I could recommend is since you have OnDemand you can use the Workout Blender and use that to create your own workouts. Since you want to do 20 to 30 minutes, you can go to her different workouts and piece to gather what you want to work on with your upper body. But, mine suggestion always have like a five minute warm up and a five minute cool down like stretching. Don’t ever leave a workout without stretching your muscles afterwards. Also, if you are seriously thinking about your upper body do like one day shoulders, chest and triceps, and then your back. I probably left out some. But, I would do like shoulders like day 1 and then do something different on day 2 and then day 3 chest and triceps and day 4 something different and day 5 back area and day 6 something and day 7 rest or do stretching or yoga. If you continually do like shoulders for seven days a week you are going to get sore and hate it because you will get tired. You need to rest the muscles too. One thing about Cathe is her upper body workouts are awesome.
For the days that are different, you can do something like take a walk or go for a swim. You said you had something wrong with your knees. Swimming you use a lot of upper body and it is easy on the joints. I am not a trainer, but maybe it might help. Also, I have found for me, I have knee problems doing squats like Cathe does in her dvds with dumbbells actually helps my knee pain. I don’t use the weight she does, but it does help believe it or not. Also, by doing the squat it self may help.
Hope this has help,
Jen Den