Advice I gave a colleague a few months back:
I took the family to GWL in Williamsburg on Dec 30. Booked one night and it was enough as you are allowed entry into the park beginning at 1pm on the day of your arrival. So I timed our arrival accordingly. You also have access to the water park all day on the day of departure, so I also booked late check out which was not necessary as I'll explain. When I do it again I will book the same kind of room - there are 4 of us, so I got 2 doubles (or queens, I forget) with a pull out sofa and no late checkout. Day of, arrive before 1pm to check in and put your suits in a different bag in case your room isn't ready, ours was (the FAQs on their site are pretty insightful). Before you leave the park on day one, reserve a table for day 2 (see if you can reserve a table for day one too - I'm not sure you can though). I think it was $40, but well worth it cause you don't have to hunt down chairs (there aren't that many) and it's just nice to have a "base." Breakfast in the onsite restaurant was the typical overpriced buffet. After breakfast take your stuff out to the car and enjoy the park. Lunch at the little shack in the water park isn't bad and not too overpriced. The reserved table also comes with a locker so you can store your change of clothes there - or in your car they let you come and go freely. There are changing areas - nothing special in the park so you can change before you leave. There isn't anything "extraordinary" about the park (like Disney or anything), but it is clean, nice and lots of fun. The rooms were actually bigger than I expected and had a nice fridge and microwave. We brought snacks, drinks and other food. No need for towels as they keep them well stocked. We were there on a holiday Monday but if you have the time, I'd recommend taking days mid week in order to really have fun w/o a lot of standing in lines, which weren't really that bad for the big slides - I think the longest we waited might have been 15 mins, but it's all good and ebbs and flows throughout the day, but you will notice lines getting longer in the afternoon when it's the convergence of those just arriving and those sticking it out til the last minute....