We all know Pavel's "Grease the Groove" works from experience; this is just scientific back-up.

If you want to be in marginally better shape, set between 4 and 10 timers every day, and do one set of push-ups, or squat jumps, or both, when the timer goes off. If you're not used to this, do half push-ups or lower-down push-ups. For the squat jumps add some weight (i.e. dumbbells) over time. You can play around a lot with push-ups and squat jumps, both to make them more interesting and work different muscle groups.