I wouldn't use sphagnum moss because gray tree frogs are aggressive eaters and could catch a mouth full and become impacted. I have some nice sheet moss in my tank and it's working really well. Your substrate should be the eco earth, that is a great one. If you want live plants, make sure to put in a drainage layer. I have a lot of nice plants, I have 2 ficus and a bromeliad that are definitely my tree frogs favorite plants. I also have a few viney type plants in there and some ferns.

If you want to put in a cool water feature, you can always research how to build a waterfall in your tank. I purchased a premade waterfall but then had to make some adjustments because it didn't work correctly. Remember that height is more important than anything with your tree frogs. Mine do not touch the ground for any reason except occasionally during hunting, they like their arboreal space. If you want some good ideas, you should look through the vivarium thread on here. I watched that for a while before building my viv and it wouldn't have been nearly as wonderful without the advice and tips I got from the members here.

If you really want to go the bonsai tree route, look for something tropical that can handle the humidity and something strong so your frogs wont destroy it