As a toy, Stirling engines are great. You can make one yourself. And they will run on the heat from a cup of coffee. It makes an impressive demonstration to your friends when you put it on top of your coffee mug and it starts spinning.
At this point I haven't built one myself so I can't give any instructions for it; the difficulty is designing an engine that doesn't need a lathe to build it. As soon as I do build one I'll write the instructions for it here. Check the Stirling Engine Home Page for some stirling engine plans. They probably require a lot of special equipment to build them so if you want to buy one you could start by looking at the American Stirling Company which also has good descriptions of the theory of stirling engines. The Test-Tube Stirling Engine page describes how to build a very simple Stirling engine and with parts that are easy to find. The Stirling Outlet has plans and kits to build stirling engines and you will need a lathe to make them.