for dining www.pizzapub.com
Paul Bunyan's is fun (check brochures around the Dells for 10 percent off coupons):
Noah's Ark is the best waterpark but you pay about $32+ per person without coupons....so listen for radio deals or like their FB page or visit their website for coupons for starters:
I'm not sure what else you'll find for cheap specifically. Walk around and explore downtown... you could go in on old time pictures at Old Abe's:
http://www.oldabes.com/ It would be cheaper to split since you're a group, if you get a smaller photos pkg. or some such thing.
explore Ripley's $13/per person
there's the Lost Voyage Boat Tour, Jet Boats like you mentioned or the Original Wisconsin Ducks which are great too. You'd have to check prices but I know they're not "cheap" so look for coupons/deals on their websites or in the brochures you find around town. You can buy coupon books but for a day in the Dells I don't see this as really moneysaving if you're paying into it and only there one day. But we have yet to try it either way.
The waterparks generally don't open till Memorial Day weekend. I see you plan on being there in the next few weeks. So keep that in mind as well unless you were thinking strictly indoor passes to the Kalahari or Wilderness, etc.
Edited: 2:15 pm, April 18, 2012