This is an neat little solar robot that would be cheap and easy to build.
“Make your own solar powered robot at home using things you probably already have. Transistors, resistor, capacitor, solar battery and flashing LED are available at any electronic store, if you don’t already have them. Solar cells out of calculators work as well, you might want to use more than one since they are really small. If you do, connect the positive wire from each solar battery together. A small motor should be used like the ones in walkmans. Solder everything together and watch it come alive. Put it under a light and let it charge, it should take couple seconds for each burst of energy. Works best in sunlight. There are two solar panels connected in parallel providing more power, and there are two 2200uF capacitors connected in parallel giving 4400uF.”