A few years back, I posted some notes about an origami workshop that I had run with some middle school students. Last week I had the opportunity to run origami workshops with similar groups of students, using many of the same models I mentioned before (including the hopping frog).
One nice model that I used this time that is not mentioned in that other post is the multiform (aka windmill/pinwheel) - a flexible hinged surface from which several simple models can be folded, including the windmill, the house, and the pajarita, and which can be extended to form the masu box and others.