Oh, yeah, I haven't done this in a while!
Top 5 Strength Exercises For When You Have No Equipment and/or Only 20 Minutes:
5. Push ups (military, knee, bench, wall, whatever you can do!)
4. Tricep dips (when it gets easy, lift one leg straight out in front of you)
3. Walking lunges (keep going until your quads quiver)
2. Bicycle (for abs)
1. Superwoman (lay on your belly and lift all your limbs; for your back)