If the battery life's good (and since it'll have an AC adapter ption), you can include Kid Chameleon. It's one of my favorite games on the system, and I think a lot of people like it too (taking from brazilian gamers, at least).

I don't know if it's one of the best sellers on the system, though. SEGA lets Tec Toy use it on their MD (and their handheld too, IIRC), so I guess it wouldn't be a problem.