Ever though about building a tree house? I just started off with the most simple design you can think off.
Choose 2 trees and connect them with 2 solid beams (6 by 2 inch)
Make 2 triangles out of lighter beams (3 by 2 inch)
Put the triangles on top of your solid beams, attach them to your trees if possible and connect them
You will have a basic tent-style treehouse structure.
Today I show you step 1. It's my first post so if things are not clear/visible/etc..., please tell me
The most classic way of attaching a beam to a tree is by using 1/2" x 4" lag screw straight through your beam and into the tree. This is the kind of screw you give a hard hit and then screw and then hit again, screw again, etc... It's really ok to do so but i don't like screwing into trees. I do something else which i first saw in Thailand, you use huge bolts and nuts left and right of the tree and connect your beams like that. I show here a picture of what i did. I did the above twice to be 100% sure my beams are not going to come down. So the beams are already resting on 2 other small beams before I put another 2 extra bolts in my 
main beams. It looks all a bit strange on the picture because my bolts were double the needed size and i had to put a lot of extra wood pieces to make it fit.
Secondly, a picture of the beams hanging in the air. Careful, this is not the beginner's way, your trees should be separated by 6 to 8 feet if you do it first time, my trees are 20 feet separated and my beams are not 6 by 2, only 4 by 2. This is asking for trouble :-) but I plan to put some middle posts, I have a lot of fallen trees around which i will use for that. Will put a 2nd post soon with hopefully steps 1 to 3 finished. I asked my children to do some acrobatics on it so you might believe this is actually not going to come down so quick :-)
Take care
bub