Low impact workouts?

Looking for some suggestions on low impact workouts for therapy/recovery.

Some back story-

Friend of my received a medical discharge from the Air Force several years ago after they couldn't figure out the medical condition that was causing him to not meet the PT requirements anymore. He was quite fit back in college but after the discharge he fell into some depression, gained a shitload of weight and been living is his parents' basement ever since.

A few months ago (on the advice of a massage therapist of all people) he had some tests done and tested positive for Lyme disease. He started antibiotics but I convinced him that getting on a healthy diet and exercise would help speed up any recovery. I know he has some motivation now to get his shit together so I've got him coming over every day to use an exercise bike. I really think he should expand on that and start some other low impact exercises, but I have no idea where to go from here.
Any help would be appreciated!

 

Does he live in an area friendly for mountain bike riding?

 

The pool burns calories and is low impact on joints and such.

 

Treadmill at highest incline at 3MPH is equal to running with it flat at 7 MPH for the same amount of time. Good cardio to burn fat without the pounding.