Mario would be my suggestion. It's fun, non-threatening, and it's something the adults can enjoy too(assuming they like to have fun).
I would not recommend Monkey Ball. I'm a big fan of Monkey Ball 2 on the Cube(I still play it on occasion), but Banana Blitz on the Wii was a letdown. Now I've heard the single player mode is actually fun, which is something I didn't get to try, but the multi-player, which is what you're most concerned about, was pretty hit and miss. As a matter of fact, out of the 2 dozen or so mini-games me, my brother, and ex-wife played, the majority of them weren't any good, with many of them have very poor and frustrating controls. There are a lot of mini-games on that disc, so it's possible there might be some good ones, but I decided not to risk my money on the game after trying it out. My brother, who is another Monkey Ball fan like myself, actually traded the game in, which is something he doesn't often do.
Although I've never tried the multi-player, Rayman Raving Rabbids is a fun game they might enjoy. The sequel, which I'm not sure if it is out in your country, is said to have a much greater focus on multi-player than the original. So Rayman RR2 might be a better choice for them than the original.
"I feel like I could take on the whole Empire myself."
