I'm finally up to teaching sound in my physics classes. This week we did a number of sound related labs/activities. One was determining the speed of sound using a long tube and a flat surface.
The tube (1.5 - 2 m) is stood up on a hard surface (tile floor). Using Audacity you can record a snap at the top of the tube and the echo of the snap off the floor. It's really quite cool. Once you zoom in quite a bit it looks something like this. I've written up fairly comprehensive instructions. If you have any questions or need further clarifications feel free to respond to this post.
Audacity Echo.pdf (Sorry, my link is broken. I'll fix the link to the document soon. Just drop me an email and I'll send you the directions)