[RK]

You can do a lot for 2) with Slinkys or other long springs and a stopwatch to help understand waves, frequency, etc.

There's also a cool outdoor experiment you can do to measure the speed of sound by clapping pieces of wood/books together in a constant, even rhythm such that you hear an echo between each clap (so you hear clap - echo - clap - echo). By measuring the distance to the wall, and timing the sequence for a set number of claps/echoes, you can get a good estimate of the speed of sound. You can also make a game to see which "team" can get the best measurement. It's a very cheap demo, but it utilizes a lot of concepts of physics and physics experiments.