Balloon Rockets:

Provide students with these materials

Large Balloons, small balloons, paper clips, masking tape, scotch tape, duct tape, straws, string, paper, tacks, glue gun, sticky tack, balsa wood or bass wood pieces 1/16 by 1/16, string, coffee stirrers, popsicle sticks, rubber bands, clothes pins, and whatever else you can think of.

Place a string across your room. Tell them that they have to make a "rocket" that will follow the string.If they use any item from the store of materials it cost one. You can modify this but exceptions I use:

Big Balloons cost three

Rubber bands, coffee stirrers, and paper clips are three for one.

String is one meter long and tape is 6 inches long except duct tape which is 3 inches long

Have students make a rocket that will cross the room. They can use any of the material and they can use more than one of each. They must submit a diagram and a written explanation of their rocket before they even get the material. There drawing has an itemized cost of materials. Don't let students experiment. This is off the drawing board and into the fire kind of thing.

Have a contest and compute the Cost/Meter of their rocket. The one with the lowest ratio wins.

Notes:

Don't exclude these creative solutions but don't tell beforehand.

A propeller

A single straw on a balloon.

A slingshot type launch

Holding the string at a high angle and letting the rocket slide down.

Letting the rocket follow the string and not be attached to it.