There are several water parks, indoor theme parks, boat tours, camping, shows, excellent restaurants, etc. We were here for a family reunion, so we didn't hit any of the water parks. We did an indoor theme park, went on a duck boat tour, and enjoyed the time at our resort. At the resort I took my kids out on the lake in one of the resort's rowboat which was cool!
I will say that the Wisconsin Dells is expensive to visit in the summer. All hotel/resorts are very high priced for rooms, tours can be expensive, and water parks are quite expensive too. Just be aware of that. You can have a lot of fun on the lake and river without spending much money, but you can also have fun spending quite a bit of money too! Regardless of how much money you wish to spend, there are plenty of things to do to keep yourself busy and have a great time.