Hammer Down Catapult Glider - http://ideas-inspire.com/hammer-down-catapult-glider/ this is a small free flight glider that can be catapult launched. The wing is created from foam found in a meat tray, tail surfaces are from foam plate, fuselage reinforcement is from thin cardboard, and a bamboo skewer is used for fuselage boom. There are plans that can be downloaded in pdf format from the website.
Foam Jet II - http://ideas-inspire.com/foam-jet-ii-catapult-glider/ is a glider that resembles a jet aircraft built from foam plates, foam meat tray material, and thin cardboard. It can be catapult launched. Plans in pdf format can be downloaded from the website.
Fantastic Foam Flyer - http://ideas-inspire.com/fantastic-foam-flyer/ rubber powered model airplane built from foam plates and a small amount of balsa. Can be flown indoors or outdoors and capable of long flights. Complete building instructions and templates in pdf format can be downloaded from the website.
Model Wind Turbine - http://ideas-inspire.com/model-wind-turbine/ model wind turbine is constructed from cheap wood and thin cardboard. An inexpensive electric motor is also needed. Students connect to voltmeter which can be very inexpensive also to test efficiency of their wind turbine.
Mousetrap Car Construction Article and Plan - http://ideas-inspire.com/mousetrap-car-construction-article-and-plan/ mousetrap car constructed from balsa wood. Axles are created from coat hanger wire and hubs are from tarp strap material.
Simple Electric Motor - http://ideas-inspire.com/simple-electric-motor/ simple electric motor built using a small amount of wire, plumbing brackets, and a small board.
Syringe Hydraulic Arm - http://ideas-inspire.com/syringe-hydraulic-arm/ Very popular project to teach basic hydraulic concepts constructed of inexpensive wood, syringes, small tubing, and small bolts.
Water Rockets - http://ideas-inspire.com/water-rockets/ in this article there are general construction ideas for a simple water rocket and a template file for the fins that can be downloaded in PDF format.