My son plays a lot of RPG games like Final Fantasy and MMORPG games like World of Warcraft and Runescape, and he likes to make up stories in his head, but so far he hasn't put any of them in writing and if he would just do that I might consider them somewhat educational. I do think these games excite his imagination, and I always thought it was kind of strange that he doesn't choose to read much fiction when this looks to me like interactive fiction. Although he does like some historical fiction and he really likes Japanese manga, he often prefers to read history and science and technology and magazines like National Geographic.
My son also likes simulation games and I think these are more educational than RPG games. He just bought Sims 2 and is saving his money to buy the Sims 2 Open for Business which is recommended for teenagers and lets players learn the challenges of entrepreneurship, from hiring and firing to devising an effective marketing strategy. He also liked playing Capitalism II and several of the Tycoon games. I think these games can be very educational. I think my son learned a lot from them in a fun way since they are interactive.
My son used to play chess on a video game. I don't play chess so he doesn't have anyone else to play with. He does have one friend who likes to play chess, but when he comes to visit there are too many other choices and they usually end up playing some RPG game because they think it is a little more fun than chess.
