We just spent 2 nights at GWL in mid Nov for both my kids B-Days. Whan fun! If you haven't been there in 10 years prepare yourself to be amazed. Also, prepare yourself to WALK! It is quite a distance between their 3 separate waterpark areas but definitely worth it! Also, they have a b-day package (I think it was called the "Wild west pkg?" that includes room decorations, cake, party favors, candy, pizza and pop for 4 along w/coupons that can be used for their lazer tag or the arcade. They also decorate the room door before you get there. My kids (turning 10 and 13) thought is was very cool
As far as price goes for the B-Day package its pretty much a break even deal and everything is ready when you check in!
I have never stayed anywhere but the waterpark resorts while in the dells but our family did stay at an AmericInn in northern Wisconsin (on the shore of Lake superior) this summer. For the price we were quite impressed. The free continental breakfast is always a nice extra. I would try to search the internet for a good deal...that is how we decided on the AmericInn...I found a good price over the internet. I hope you have a great trip!