My tree has a cardboard tube "skeleton" for the trunk, made from concrete form tubes. The large branches are made from cardboard mailing tubes, and the smaller branches are made from foam pipe insulation and pool noodles with a length of 18g wire inside to give the branches shape and structure.
The bark is made from Great Stuff brand expanding foam, sculpted with a saw and finished with three different colors of latex house paint.
The whole thing probably weighs around 10-15 lbs -- it's incredibly light. The branches are suspended by 25 lb fishing line threaded through eye bolts in the ceiling.
The trunk is supported by a wall. For this project I wanted the tree to appear to be growing up inside the wall and ceiling, so it's basically a half-tree, open on the side that's touching the wall. For free standing trees, you'd want to build a solid support system inside, but for a tree that's leaning on the wall, just a couple screws are sufficient to hold it up since it's really really light.
I don't have kids or dogs, so I'm not super concerned about the durability of this tree. The surface is spongy but solid -- the latex paint keeps it from crumbling and gives it a nice touchable texture. If you're making yours for a playground environment or a kid's room, you'll likely want to add some hardeners to your paint or use different materials so it can be climbed or poked or grabbed with grubby fingers or chewed on without damage.
There are products such as Hard Coat that you can add to latex paint to give it a hard, durable surface. I haven't tried this myself so can't vouch for how well it works in this application, but if you're making this tree for a kid's room it's worth a little research and trial-and-error to create something that will last for years.
Planning & Layout
What Kind of Tree?
Spend some time outside, walking around your town. Find your favorite trees. Touch their trunks. Sit under them for a time. Fall asleep listening to the rustle of their branches. Say hello and learn their names. The trees are alive and aware, and will answer any questions you have, if you take the time to listen. Do this until you know how to proceed. It shouldn't take long.
My yard is full of oak trees, so it felt right for me to create something as close to an oak as I could. It was easy to know I was on the right track since I had examples right outside my window, and now that it's finished, it feels natural and "right," part of the landscape. It resonates and connects the inside of my house to the outside.
I've confused all the neighborhood birds, who are continually trying to fly through the window and make their nests in this very appealing new tree. I'm not sure whether this upsets me or delights me.
Choose the best spot for your tree. I took a picture of my preferred corner and used my iPad's markup tools to do a few sketches for how I envisioned my tree to lay out. I didn't plan too tightly, I just needed a general idea of what trunk diameter I wanted and where the main branches would go.
I didn't want to block the upper windows too much so planned on an 8-10" trunk -- which works out perfectly with the size of the concrete form tubes available at my local hardware store. My ceiling is 12' high, so three 4' tubes stacked on top of each other filled the space perfectly.
Decide early where your Tree Ent's face will go. I wanted mine at eye level for adults, too high for children's poking fingers to reach.
Also remember you'll want a power outlet near the base of your tree.