We were at GWL Williamsburg for Snowland in December... The lodge was decorated beautifully for Christmas, the water park was empty (more guards than guests!), and our kids had a blast. Everything there is centered around children... It's kind of like a paradise for them!
Besides the waterpark and arcade, my boys enjoyed the "Wolf Walk" in the morning which was a cute nature program. They loved the Clock Tower show and pajama storytime, too.
When you check in, you'll get a schedule for crafts in the Cub Club. They do a different craft every half hour or so... Some are free, others cost $2-3 each. We went the club expecting it to be a little more organized than it was, but the kids made some cute Christmas ornaments and snowmen there.
As other people have mentioned, the food at GWL is HORRIBLE!!!

They hyped a breakfast buffet with Wiley the Wolf at the pj storytime, so we went there the next morning thinking it would be fun. Don't be tempted! There was NO Wiley and the buffet was really, really bad. There are tons of family-friendly restaurants on "Restaurant Row" just a couple minutes down the road... You could probably get a much better meal there. If you really don't feel like venturing out in the morning, though, Starbucks and pastries at the BearClaw cafe are a good bet for bfast. Pierce's BBQ is great for lunch. We also like Sal's Italian and Shacklefords (spelling?)for family-friendly dinner food in Williamsburg.
One thing I'd recommend is renting one of a waterproof cameras from the gift shop. They take great digital pictures, and you can wear the camera with the attached wristlet right into the pool. We got so many good shots with ours. Before we checked out, we just had them loaded onto a Great Wolf Lodge CD in the gift shop. I'm sorry I can't remember the cost, but this was a great souvenir. The CD has all of our photos in a digital slide show set to GWL music and my kids have watched it again and again.
If you like doing pressed pennies, there's also a pressed penny machine in the lobby like they have in Disney. These also make fun souvenirs.
This is probably way more info than you wanted, but we were there so recently that it's all still pretty fresh in my mind. Hope you have a great time!
