It increases your adrenal reserve and builds muscle without draining or crashing.
Start with gentle yoga, move up to power yoga. After a year of gentler practice (via http://gaiamtv.com)
I now do 75 - 90 minutes of hot power yoga at a local studio, 4 times a week, and it is raising my energy and vitality to brave, new levels. Feeling better than I ever did before AFS!
Your second best bet is gentle aerobic exercise. I find cycling on relatively flat terrain to be an excellent, low impact form of AFS exercise.