Don't get turned off by the touristy stuff, (go carts, water slides, etc) along highway 12 and the interstate. The real reason this area became an attraction was the Dells themselves...beautiful rock formations along the river that can only be seen from a tour boat. We prefer the classic tourboats to the "ducks" which are old WWII amphibious rigs that are noisy and smell like diesel exhaust. The classic boat tour is restful and peaceful. On the upper dells tour (above the dam) they stop and let you get out and explore for a short hike before continuing on. Also the scenery is prettier than the lower dells tour. The tours are narrated by the "boat captain" who is usually a college kid that does a good job, but tells some of the most corny jokes you've ever heard.
There are some great restaurants if you do some searching. As others have said, Circus World is great, as are the two nearby state parks. We thoroughly enjoyed house on the rock...the house itself was nothing special, but there are building after building of collections of every kind....it was a hoot!
A short drive away are some great winery tours, and the countryside around that area is just gorgeous. Again,,,,the area on the strip is very touristy, but we enjoy driving through to look...where else can you see a 5 story high real wood Trojan horse, or an upside down White House?
ENJOY!