I've got lots of motorized, plastic robot models for kids. Putting these together develops building skills and the final result is a fun toy. I think everybody should build at least one or two of theses during their life. Starting young develops skills that are useful for a lifetime.
While most of these are battery operated, I do have kits that are powered by solar cells, wind vanes and fuel cells. All of these make great summer projects and may help your child get a headstart on next Fall's Science Fair project.
All of these are available online. Let me know if you have trouble finding them or want a recommendation. If you prefer to pick up in the store, let me know your preferred days and times for store hours.