Several years ago I switched over to rechargeable batteries because kids are battery gluttons. They still manage to run through them like water, and occasionally lose them. But that's only one reason I like this 6 in 1 Solar Robot Kit from Robotikits. The biggest reason is that I want to play with it myself.

This is advertised as "educational" because of the solar power, which I suppose is a good thing. Kids know about solar power, but outside of their pocket calculators they don't get to play with solar power much. This little pocket-sized generator runs the kinetics of the robot outside on sunny days, or inside under a 50-watt halogen lamp. The parts go together to form six different solarcovershapes of robot: a car, a puppy, an airboat, a plane, a windmill, and another plane that flies around a pole. No screws are involved in the assembly, so it should go pretty fast. And when you get tired of one configuration, just reassemble it into another shape! I'd like to see the puppy walk most of all. I think the cats would really enjoy that.
The 6 in 1 Solar Robot Kit is £12.99 in the UK and $19.99 in the US.