The 4M Salt Water Power Robot by Toysmith looks like a totally awesome tween gift, and my almost-eight-year-old qualifies. (Eep!) It’s a mini robo-racer that you put together fairly easily, then power it up with ordinary table salt and water. It seems that the solution creates a reaction with carbon and aluminum sheets to make it go, which is an awesome way to teach science in a way that video games simply do not.
Unless you have a kid who’s proficient in taking them apart and putting them back together. In which case, I’d like to borrow her for a few days. –Liz