First of all, try your best to integrate the house design with the tree. Never ever just put a small cube on top of the tree, that looks terrible. A good place to build a treehouse is in a jungle biome. Use materials that go well with the jungle theme, such as jungle leaves, jungle logs, oak wood, glowstone, fence etc. Sort of something like the following.
Of course this can be much better, I just did it in 5 minutes. Nevertheless, hoped I helped