If you're feeling ambitious, you can make a very cool tsunami demonstrator following the directions found here.
If you're not up for such a task, you can make simpler models using 2 liter soda bottles. While this simpler model probably doesn't have the same impact as the fancier versions, it does have the added bonus of allowing students to take part in its construction and manipulation.
Fill the 2 liter bottle with about 2 inches of gravel. (I used sand because I had it on hand, but gravel works MUCH better).
Then pour about 250 ml of water (about 1 cup) into the bottle. Cap it tightly.
Gently lower the bottle to its side, so the gravel forms a slope at the end of the bottle (you'll see that the sand doesn't work so well at this point, in the picture below). The gravel slope represents the sea floor and then the beach. The water represents the ocean.
Use the palm of your hand to smack the bottle cap (i.e. the end of the bottle opposite the gravel slope), to generate a wave.
Observe the wave formation and the way it crashes upon the gravel. Also note the way the water sloshes around on the gravel following its initial crash - the danger of a tsunami extends beyond the initial landfall.