If you have the money, and can afford it, I'd definitely go for Rock Band instead of Guitar Hero. We got GH for the Wii and the kids enjoyed it, but once we got Rock Band that's all they've played since then. Plus 3 (or more if you have 2 guitars) can play it at once, unlike GH and they get to sing and play drums as well (I have a percussionist and a singer in the family so it's great - no real guitar players).
I'd recommend the Wii Fit for you if you're at all interested. It's helped me lose 13 of the 15 pounds I want to. I don't use it that much for the training (although with winter coming I may more), but just having a place to weigh in daily and track what I'm doing helps keep me in line.
Cranium Kabookie was a big hit for our family - it's the Wii version of the board game Cranium. Lots of fun for everyone.
And of course Super Smash Brothers (whatever the new Wii version is) - that's a huge hit whenever their friends are over!