They do at least triple duty. First, as a safe place to play, they're where the action is. They form a backdrop or support for plants. And often they're eye-catching. Kids run to them, adults wish they were small enough to climb up or crawl into them.
Some structures are colorful, whimsical, and humorous to look at.
I'll skip major construction projects like decks, swimming pools, Victorian playhouses with gingerbread trim at the gables, and the like, in favor of inspirations you're more likely to try. Try bamboo, angling the poles like interlocking fingers. Map the rooms out on paper first. For windows, break up the walls by planting peas instead of corn. Willow or other bendable twigs, such as dogwood, can be fashioned into rustic arches, walls, nests, and free-form sculpture with the woven technique known as wattle.