The Burpee Pullup
Find a playground, a soccer goal, scaffolding, a tree branch or just about anything taller than your overhead reach. Drop to your stomach and then pull yourself up until your chest hits the bar. Repeat 10, 50, 100 times--whatever you need to do. Every single muscle in your body will get worked and you'll be cashed out much sooner than you'd expect.
This is a great way to modify your body composition, get stronger, develop endurance and become more generally competent with your body.
Difficult movements make for a hard body and strong mind.