An old parkour friend of mine started doing 10 pushups on the hour every hour he was awake. Then after a week, he would move up to 11, 12, etc. When he got to 25, he started over with inclined pushups. If you're into rock climbing, you could instead switch to tri-pushups or fingertips. It makes for a really good break from whatever you're working on during the day.

I could never quite get on it due to my classload, but he does probably 100 handstand pushups a day.