Mellie -
I echo the other tips. Since you are in MD and driving, consider bringing a cooler filled with snacks, sodas, etc. That's where you can save some real money. The room should have a fridge, but even if not, you have your cooler. This will also allow you to bring juice, milk, etc. (note to GWL management - charge $.75 for a soda and people won't bring coolers - and you'll make more money...)
Consider bringing a book, some playing cards, a radio, puzzles, or other games appropriate for those who may not want to be in the water all the time. One of the nice things about the waterpark is that it is closed, no flies, no burning sun, no rain - so you can sit and play and not worry about the kids running off, or worry about feeling uncomfortable. So get the room's money worth and just hang out poolside.
Another suggestion is to bring a portable DVD player (and movies) to hook up to the TVs in the room. Bring some movies, microwave popcorn and your evening is set. However, I would also recommend a walking candlelight ghost tour in Williamsburg if the kids are old enough. Great way to spend an evening and not too much $$$.
Bring your own coffee cups and coffee/tea. Heat the cup and water in the microwave and have a nice beginning/end of day drink.
As others said, it can be a hike back to the room, but it is all indoors and is not too bad. Our strategy has been to bring everything we think we might ever want - then load it all into the room - then if we are poolside and want it, we go get it.
There are loads of shopping/eating places within 10 minutes, and some are next door - but none are within walking distance. Be prepared to drive to get anything, even from the neighboring
Walmart