This gravity/motor propelled wheel takes about 30 minutes to put together. Make sure and modify it to your wants.
Although I haven't tried it, other building kits might work as well or better.

This Is my first instructable so feel free to let me know of any problems or better ideas.

Step 1: Parts

You'll need:
1: fischertechnik wheel
1: cheap remote control
12: curved fischertechnik beams
6: size 60 cross beams
1: short axle
19: short pins
4: size -120 cross beams

Step 2: Assemble the Wheel

Hook together 6 of the curved beams into one round, repeat process so now you have two full rings.

Step 3: Hook Them Together

Now take your short size 60 crossbeams and attach the curves together with 12 of the short pegs

Step 4: Attach Motor

Using size -120 crossbeams connect the two edges, making sure to attach with pegs all but the fourth hole which is where the short axle goes.

Then connect the wheel to the small axle and the motor to it. Now set on the floor and switch it on and watch it go.