I'm sure that there is a timeframe that docs want you to wait (I don't remember what it was) to do the really heavy exercising but there was a gym at the hotel that I stayed at after my transplant and they wanted me to go there almost immediately and at least walk on the treadmill. I didn't though, I just walked around the hotel.
I can say that my first really big exercise was just a walk, but it was a huge one after I got home (about 8 weeks after the transplant). I walked over a mile to the park, played with my son at the park, and then walked over a mile back home. Yeah, I was sore after I did that but it felt great!
I am now 2.5 post transplant and there are a lot of things that I have done, including run, walk, Wii Fit, and chase after my 3.5 year old son and the baby that I watch occasionally...stuff that I would have never been able to do before.
You exercising routine will most likely be dertmined by what the docs tell you that you are limited to and what you can tolerate post transplant.