Resistance band exercises
There are hundreds of ways you can use resistance bands in your workouts from assistance training, to adding weight-lifting resistance and stretching / rehabilitation exercises.
If you add handles to your resistance bands you have a portable gym, which weighs in less than 3lbs that you can train the entire body. The further the bands stretch the more resistance is added to the exercise, maintaining muscle tension throughout movements and typically adding the most resistance to portions of the exercise that typically get easier.
Resistance bands aren't simply used for building strength though. Anchor your bands to fixed point and use the bands as part of ballistic running drills to improve your acceleration, speed and coordination skills.