I don't know what it is about tree houses that captures the imagination but they seem to bring out the creativity in designers. Students might be inspired to be a bit freer with their designs if a project in architecture was to design and build a model of a tree house. The popularity of the movie Avatar is also a good motivation for thinking of designing in trees.
Getting some branches to stand in for trees provides a structure for the students to design around. Cut 10-12 inch sections of branches that look like small trees and attach them to a platform. Have the students design models out of wood and other materials appropriate for a tree house (probably no brick or steel designs although who knows).
As in any design project, the designs should obey laws of gravity, tensile strength, compression, etc. or else they remain in the realm of fantasy illustration. Good design is imagination tempered by an understanding of physics. The Floating Mountains of Pandora in Avatar are a good design project for movie designing and sci-fi illustration but a bit far-fetched for an architecture project.
A quick search online will turn up hundreds of inspirational designs under the key word "tree house."