Okay, the "Plant Nut" in me is coming out. I saw an article on Vocalpoint that tells how to grow an Avocado Tree from seed. This is something I plan to try right away. I'll let you know how it turns out.
After you eat an avocado, place three toothpicks into the seed, each going in a different direction. Suspend the seed in a cup with the pointed end up, and add water until it covers the bottom of the seed. Place the cup (and seed) in the sun and water it for as many days as necessary to grow one long root. Then, remove the toothpicks and plant it in a large pot. In two months, your avocado tree should be three feet tall with massive leaves! It's a great home science project that your kids will love.