What started out as a boring, ecru white room with a standard Ikea platform bed became an indoor tent space for my tween son who loves blue camouflage. We used the space underneath to create a desk with a pegboard organiser.
One needn't feel like they need an amusement-park-worthy tree fort indoors to keep kids happy. A tent-like canopy over a bunk bed or loft is really all it takes.
YES! Think tent, and add something along the walls near the ceiing. Perhaps wall paper, or maybe a shelf like you saw in some of the houses you toured...