you could head up to the confluence of the Mississippi and Missouri rivers. It's way out of the way, and the sign says they close the gate at dusk, so if you go, you would probably want to head right over there after work. You go north from St. Louis, turn off the highway and drive 4-5 miles on a small paved road then 3-4 miles on a gravel road.

There is a state park there and they have some signs with information about Lewis and Clark (they started their expedition there), but mostly it's just the place were the two rivers come together. I thought it was pretty cool standing there on the point and contemplating the massive volume of water flowing past on both sides of me. I didn't see another soul while I was out there. It was very peaceful.

Anyway, it would be something different than what you would normally do when visiting a city.