Yesterday I did the conservation of momentum demo with "energy sticks" in some parts of the country they use fire crackers, but those are illegal in Michigan.* The basic set up is a short section of 3" diameter PVC pipe with a small hole drilled through one side. You put a pop can in each side and put an energy stick in the hole. The explosion propels the cans out the ends of the PVC pipe.
Both cans will have the same momentum just in opposite directions, so that total momentum is zero. You can change the mass by adding some sand to one or both cans and see how this effects velocity. In order to get velocity all you need to do is solve a simple projectile motion problem. Or you can do video analysis, but your milage will vary here. The can leave the PVC at anywhere from 3 m/s to 9 m/s or so.
I forgot my pop cans yesterday so I ended up finding some Arizona Ice Tea cans. These worked pretty well (mass = 25 g plus or minus 0.5 g). Then I used a Chef Boyrdee can I had laying around as an unknown mass (ended up at 48 g). My students were tasked with finding the mass of the ravioli can. The answer they arrived at was 36 g instead of the 48 I got from a balance. My guess is the inaccuracy comes from the deformation of the ice tea can, differing geometries of the bottoms, and the slightly different diameters of the two cans. But a good problem to work anyway.
*I'm not sure if it's illegal to use them or if it's only illegal to buy them, or how the legality goes in an educational setting.
**Link to the zip file of videos pending.
I've been using pieces of James Kakalios' 
