What can a total knee replacement patient do to firm lower body?

Exercise Bike. Recumbent or regular. 4-5 days per week x 30 minutes. Slowly increase resistance. On the days that you don't use the bike, go for a long walk. The walking is ok to start about 6 weeks post-op and thereafter. A stairmaster is another good leg strength alternative.
Low to Mid Impact. I agree with things like exercise bike. Also look at swimming, elliptical trainer, etc... Anything that does not involve hard heel strikes on the ground or workout surface.