Pull ups are awesome - chin ups are good too IMO, if you can do more than 8-10 then use a back pack and fill it with books.
Do pushups with both hands and put a backpack filled with books on your back, make sure you can do no more than 8-10 of these push ups.
Sit ups are great, leg raises too if you hang from the tree. If you can do more than 12 sit ups than add some weight to your chest.
LEG EXERCISES:
Calves raise, use a backpack or similiar or do one calve at the time. Stand in a staircase and lift yourself up and down with the front edge of the foot (not the toes).
Good mornings: (hamstrings): Get the backpack filled with books as high up on your back as possible then do this exercise:
http://www.exrx.net/WeightExercises/Hamstrings/BBGoodMorning.html
watch your back with that one.
You can do some squats with a backpack too, but you'll need more weight.
DELTOIDS:
Upright rows work if you find a suitable objekt to lift. (backpack work OK)
Handstand push ups is another option.
Arnold Press probably works ok to with a backpack, have not tried it.
BICEPS AND TRICEPS:
Biceps: Biceps curl with your backpack or suitable objekt
Skull crushers/triceps extension: Lay on a table and grab the backpack and do this:
You can do hammer curls too.
http://www.exrx.net/WeightExercises/Triceps/BBLyingTriExt.html
It'll hit you in the face though :/
FOREARMS:
Wrist flexors and reversed wrist flexors with your backpack
Hope this works or you can come back and flame my ass, good luck.