Options could include any version of the Game Boy (although Game Boy Advance recommended because it is backwards compatible), there are small LED mini games that are available for cheap, not the best games, but they'll probably keep you enteratined for a while.

I find it kinda wierd why nothing Wi-Fi is allowed, I mean, not connecting wouldn't be too much of a problem, but why can't you bring something like a Switch or phone and just leave it offline?