I also highly recommend looking into a recharging dock for the controllers. Someone suggested we get one, and I love it! It keeps the controllers charged up all the time. We've had it for a year now and haven't had to change the batteries yet.
Also some of our favorite games are the "old school" Mario games from the regular and Super Nintendo systems. If you are interested in downloading those to play, you'll need to buy a classic controller as well as a "points" card (available anywhere Wiis are sold). The old games are purchased with "points" and they download straight to your wii system if you have wireless internet in your home. No need for disks or anything.
Other games that my kids love: Guitar Hero (you'll need to get the bundle that comes with the guitar), Carnival games (just the game -- no extra accessories), and Mario Kart.
But really the one we play more than anything is the sports bundle that came with the system (bowling, baseball, golf, tennis, and boxing).
Enjoy! It's really a fun system!