Wave Machine

What you need

A roll of gaffer tape. Duck or Elephant brands are the best. Which is a shame, since they're twice the price of the others, but so it goes.
Lots of wooden kebab skewers.
More fruit pastilles than seems reasonable.
Two tables or chairs of the same height.
a ruler, or a number of fingers that are approximately the same width as a ruler.

What you do

Stretch the gaffer tape between the furniture. It's probably best to try this with a 2 metre span the first time, but I've done spans of up to 4 metres with some success. You need to stretch the tape taut, which means sticking it down firmly - go right round the table or chair if you can. Mind that varnish!
Starting from one end, stick kebab skewers to the tape so they stick out evenly on either side. This is why you need good-quality tape - you'll need strong adhesive to hold the skewer. A damp day will ruin the adhesive, too - if your skewers fall off, this could be why.
Space the skewers about a ruler's-width apart. The exact spacing isn't critical, so long as they're fairly evenly-spread. Keep adding skewers until you've reached the end of the tape.
Now put a fruit pastille on each end of each skewer. You might think you'd only need twice as many sweets as you've used skewers, but mysteriously they tend to disappear. Further experimentation is required to ascertain precisely why. Much more experimentation. Way more, in fact.
However, be careful when pushing the sweets onto the skewers. Careful in case you accidentally poke your finger, but also because you want the skewer to stay balanced - it should rest horizontally. You may need to push things around a bit to make sure your wave machine stays balanced all along its length.
Once that's done, your wave machine is ready. Hold the skewer at one end, swiftly pulse it up and down, then let go. You should see the pulse travel the length of your tape, reflect off the far end, and return to the start. If you drive the end continuously, you can set up a standing transverse wave, too.

What's going on

Transverse wave machines like this have been around for years, but they're normally very expensive. Not only does this version cost far less, it often works better too!

The tape acts as a torsion spring, but - and this is key - one with a very low spring constant, and very low damping. The fruit pastilles and kebab skewers provide a high angular momentum, too. The result is a wave that travels down the tape at a surprisingly slow speed, as the tape struggles to return to its central position.

With a little practice, you can demonstrate not just standing waves, but things like wavepacket dispersion too. Now that's something you didn't expect to see done with a fruit pastille, right?

(the photo, by the way, is of an early prototype of this demo. It works quite well with elastic, as you see here, but the gaffer tape version is simpler, more reliable, cheaper, looks better, and is easier to set up)