Speaking of the PlasmaCar (see previous post), there is another ride-on toy my kids absolutely love. It is the Flying Turtle. Our neighbors own a couple of these and my kids ride them in the driveway any chance they get - around and around and around. It is another kid-powered toy, again moving the steering handles right, left, right, left. You can sit on your butt, like this picture. Or you can ride like my boys - on your knees with hands on the steering handles and then you have a free foot to push you off and give you even more speed. These are also a total blast in a gym because you have so much room to ride!
They come in a few colors and are usually $60 - $70. Another toy where kids have to use their own energy and not rely on a motor to move them around!