Though it is not a waterpark pe se, the Hays municipal swimming pool is above par. It's got nice slides, a lazy river to float down in a tube, and a lap pool for older people.
As mentioned there is old Fort Hays for history buffs, and if you'd like a walk downtown, there are historical markers that tell the story of the early town. There were a dozen brothels on one street back then!
Most of the new motels are on the north side of town, along the interstate, but downtown is a less than 5 minute drive away. Also on the south side of town is Frontier park. It's a good place for a nice walk, a picnic, or a game of frisbee golf. Big creek meanders through and there is a walking bridge that crosses in one place, and some stepping stones to cross further downstream.
I lived in Hays for many years and still do most of my shopping there so Hays is my suggestion. I'll admit that I'm biased. : )