Do you know any good miniature wargame that is fairly easy to learn and play?
We used to play Necromunda years ago--I'm not sure if it's still around. Like Warhammer it is/was a Games Workshop game, but I think it could suit your purposes. It came with enough "terrain" in the box to support a game. It would be well suited to a multi-player game (think of an urban gang-fight free-for-all). Only a few miniatures were needed so that barrier is low and a quick gang could be set up quickly by new players while the seasoned set up the terrain.