Forget to do extra crunches? Get abs beach-ready in the air.

Isometric tightening of your muscles is the simplest and least obvious way to exercise during a flight. Focus on a muscle or group of muscles—say, your stomach. Sit up straight, and take a deep breath. As you exhale, tighten your muscles and hold for a count of 10. Release and breathe in. Repeat 5 to 10 times. Do this every 20 minutes or so, choosing a different muscle or set of muscles each time.