Are their any online games for gifted children?

My son is gifted and loves the computer. He has a few games that he is pretty obsessed with and neither of them are at all educational. I would like to see him enjoy the computer but challenge himself at the same time. We love Carmen Sandiego and Oregon Trail. Are there any other games online and free that are similar? And yes, he spends plenty of time offline, but the computer is a reward for good behavior. I'd just like to see him using it for something that will be useful to him.

Thanks!