Another favorite of mine when I was a kid..
For those not familiar with this toy, you fill the rocket to a line with water. You then attach a small hand pump to the base of the rocket, latch it in place and pump it full of air. Now I'm pretty sure the actual instructions gave some kind of advisory as to how much air you should pump in to it but we always pumped it until there was so much air inside that we couldn't work the pump anymore (ie: maximum thrust mode). Aim up at the sky (very important!) and release the latch holding the rocket in place.
What happens next is pure science, but I don't know the actual words behind it. Suffice it to say that water + air pressure + forced point of release = launching rocket. Like any flying toy, these would sometimes end up on the roof, yours or your neighbors! There is also a good chance of getting sprayed by the "rocket fuel" as it launches which makes this a good summer toy.