what a great question...and i've got some recommendations for you since this is what i do when i'm traveling abroad and don't feel like paying to go to a gym. the key with these exercises is to utilize your body weight and maximize the wonderful forces of gravity. once your body weight is no longer an effort, either increase reps or add weight depending on your goals. Listed below are 6 recommendations. Have fun.
1. any type of push-up, whether it's elevated, one foot, one hand, or weighted. obviously these are advanced forms, but you can work your way into doing those eventually.
2. supermans for your lower back. google or RJ it to see what i'm talking about.
3. squats with tossing a weighted ball in the air. as you lunge upward, you simultaneously throw the ball up. upon catching it, you lower yourself back to the low squat position. repeat. an advance form of this is staying on your toes the entire time. also, you can toss overhand or under. i would alternate.
4. any type of pull-up or anything where you'd need to pull yourself off of the ground.
5. shoulder stands...do a head stand and slowly lower yourself down until your elbows are 90 degrees. your hands should be slightly wider than your shoulders. please do this with someone for a spotter so not to injure your neck or spine.
6. last but not least, ab work. enough said. just be sure to always counterbalance this with your lower back stuff. there are plenty of options listed here on RJ.