This appeals to me on the most basic "Swiss Family" levels. I would have dragged my husband up a tree for our honeymoon had I known it was an option.
Here are a few amazing resorts to file away under "honeymoon" in the wedding binder.

Creek and Crag's Wild Canopy Reserve: Masinagudi, Tamilnadu, India. This treehouse has en-suite facilities and the reserve serves breakfast, lunch, tea and dinner. The treehouse is situated above a watering hole so you can watch the elephants and tigers drink before you go to sleep! I think this would be my first choice.

Treehouse Village Eco Resort: Kavieng, New Ireland, Papua New Guinea. Following at a close second is this treehouse on the beach, complete with snorkeling, scuba diving, en-suite bathroom and hot shower. Meals are served in the canopy.

Cedar Creek Treehouse at Mt. Rainier: Ashford, Washington USA. Double beds, a kitchen, a bathroom, skylights, and an observation room make this eco-friendly place seem very enticing.

Takilma Treesort: Takilma, Oregon USA. A community of treehouses to choose from, some with private toilets, some without. All appear cozy and warm. Full breakfasts are provided during the high season.