This week, there is a sale going on for Plants vs Zombies. I highly recommend this game. There is a lot of replay value in its mini-games, survival modes, and even the gentle Zen Garden. It is quick to pick up, fun to play, and will keep you going to unlock all of the play modes.
Another alternative to downloading the game from Xbox Live Arcade is to purchase a disc with the game. The disc has Plants vs Zombies and comes with Peggle and Zuma. It is just $20 from Amazon, and can be sold off on the secondary market when you're done with it (unlike the download).