There is actually a store just selling model engines and gyroscope related toys.
Gyroscope.com features for instance a Sterling Engine model (photo). Stirling engines convert thermal energy into mechanical energy. These stirling engines take a temperature difference between the top and bottom plate and turn it into rotational energy, rotating the flywheel. Simply placing a cup of warm coffee/tea underneath is enough for it to work.
The Sterling Engine sells for $341.60.
Another cool Gyroscope gadget is the Japanese Gyro-Top. The Gyro-Top is produced since 1921. Outside diameter 2 and a half inches (63mm)and weighing approximately 4 ounces (100 grams).
The Gyro-Top sells for $18.61 on Gyroscope.com
See all Gyroscope.com products.