Super Monkey Ball and Super Monkey Ball 2 are probably my two favorite games on the Cube ever. I bought the first the week the Cube came out, and took it home for Thanksgiving, and that's all we did for the entire week. Mostly Monkey Target, which is incredibly addictive. My roomates used to play bowling constantly too. This Thanksgiving we rented Mario Party 4, and its pretty fun, you get to laugh at each other a lot, but the Single Player is fairly tedious. If you don't care, its's a great game, but Monkey Balls is better.
The tennis is similar to Mario Tennis, but I wouldn't say its as good. It's fun though, it just gets overshadowed by the insane amount of other great Mini-Games