Last week we drove out to Illinois to visit my parents. They live just outside St. Louis, so every visit to their neck of the woods includes a day at the amazing City Museum. It’s an art-installation-meets-all-ages-playground sort of place.
There are climbing structures (concrete, wood, and rebar figure heavily in the construction), both inside and outside:

The outside courtyard.

This connects to a giant, human-sized Habitrail mounted on the ceiling.

There are actually two gutted airplanes on towers on the courtyard—and one gutted school bus hanging over the edge of the ten-story roof!

Heading up to an airplane wing. (Yes, you get to walk on a real airplane wing!)
The climbing (inside and outside) is huge fun! Especially if you wear knee pads. (I am not kidding. You can buy a cheapo-but-adequate pair in the gift shop. Save your knees for only $4!)
There are also extensive “caves” (made of concrete) in the back of the first floor—most tunnels require crawling and a few are too small for more adults. (In other words, they’re a claustraphobic’s nightmare.) I didn’t take photos there because it’s really hard to capture images in a super dark place. (We usually bring headlamps when we come here, but this time we forgot.)
But I did photograph the slides! There are TONS of them at City Museum. Several are outside, like this one:

Super-fast twisty slide!
And there’s even a ten-story slide! It spans the full height of the building, which used to be a shoe factory. This slide was one of the chutes used to send stuff from the top floor down to the bottom. It’s not the fastest ride, but it’s TEN STORIES LONG so that makes it pretty cool.

Entrance to the ten-story slide!
There are lots of other cool things to see and do there:

It’s a Ring of Death for running humans!

Funky chairs/stools/rockers that roll around. Weebles wobble, but they don’t fall down!

No introduction necessary.

Now THAT’S some good alliteration.
One of my favorite areas is the architectural museum. It’s of less interest to the rest of my family, so I wander through there on my own when I need a break from all the climbing.
In this section are facades and decorative elements from old buildings that were torn down in the Midwest. In some cases, the demolition crews unofficially salvaged some of the pieces; in others, official preservation efforts were made (one Ohio school district found a home for all this stuff BEFORE tearing down an old building, for example). Most of the stuff is from the Chicago School of Architecture (lots of Louis Sullivan here) and the Prairie School (e.g., George Grant Elmslie).

Just a sampling of the collection.
City Museum is an extraordinary place. St. Louis has a lot of neat stuff going on (top of the Arch, anyone?). But if you have only a few hours in the city and can visit just one place, I think it should be this one.