Two things come to mind. There's a relatively cheap bumblebee rider with casters at target. Our boy has grown with this thing from the moment he could sit upright and push. Now that he's older he rips around the house on it and drifts.

Second is something to climb on. We have a product called the nugget with nevernudes (accident protection) and he's tumbled and built shops/forts out of it for years now.