Anyone with small children knows that wheels are the key to happiness. Seat one of my sons on a two- or four-wheeled device, and I can bank on at least 15 minutes of happiness. Little Tikes designed the Hop & Scoot ATV ($50) with this in mind. Its sturdy and stable so even the youngest bodies can shuffle around without fears of falling off. It looks like a bright blue motorbike, which all the little dudes love of course. With big plastic fat wheels, however, it cant get moving very fast, which moms love.
We use ours in the backyard on grass or pavement, but I think it would also serve as a great inside toy. The ATV requires assembly, but if I managed to put it together, anyone can.