An effective way to get stamps quickly is to do a higher number of workout sessions. Instead of going for one 30 or 60 minute workout (such as P90x), do a few of the 10-minute workouts instead. You'll get different challenge opportunities with each of them, and can collect a number of stamps quickly.
For the longer term, just make sure you're doing a variety of workouts to get the maximum opportunities to complete challenges for stamps. And in the process, you'll benefit from the larger variety of activities!