Nice one! I got one at Christmas:
Wii Sports - this is alright, but it soon loses it's appeal IMO. It's a good laugh with multiple people
Mario Kart - now this is amazing. You get a steering wheel with it as well, and it plays like a dream. Top laugh on your own, and also with mates/thewife/the kids etc...
Mario Galaxy - this is more of an epic adventure. There are about 120 levels, and lots to do. It really uses the remote in very unique ways. You fly, surf, run, jump, catapult mario through various 3d levels. It's really good fun, and quite addictive!
You'll also need another remote or two. I'd recommend looking around for Wii Play, which comes with a remote as well, and you also get a disc with games on. THe games aren't that great, but would keep the kids entertained.
Have fun!