I bought a puku "trolley" toy for my daughter when she was learning to walk. It's very much like the fisherprice brand but cost like $19.90 at Giant, another good thing about it is you can control the "sensitivity" of the trolley. You can tune and tighten the toy so that it's harder to push so that babies won't push the trolley too fast.
When you push the trolley, the 2 penguins infront will also make some sound. It amuses my daughter and this encourages her to walk and push the trolley. Not sure if it will work for others though.