Karlsruhe isn't the oldest city in Germany. It was only established in 1715, which is recent by European standards. It's not the biggest city in Germany, either. The total population of Karlsruhe is only a shade over 300,000. But it may be one of the more quirky cities in the country.
This is the city that celebrated the 200th anniversary of the bicycle with a major event in 2017. They have an annual rubber duck race consisting of thousands of the little yellow squeakers bobbing around. They have a mascot parade. There's no good reason not to swing by.
Waldseilpark Karlsruhe
The Black Forest is a massive forested mountain range that takes up a good chunk of Germany, including the area Karlsruhe calls home. There are a lot of myths and legends related to the Black Forest, some of which are rather horrifying. What a perfect place to set up a rope park! Waldseilpark Karlsruhe has multiple courses that give you an interesting perspective on the region.
Kindergarten Wolfartsweier
Does this recommendation actually direct you to go to a kindergarten in a small German city? Yes. Yes, it does. But this school is literally shaped like a cat. It's paws are slides and the classroom is in its belly. You aren't likely to be invited inside Kindergarten Wolfartsweier but just seeing it is more than worth the stop. Maybe ask someone about taking pictures of the school if class is in session.
Fahrzeugmuseum Marxzell
Just outside of Karlsruhe in Marxzell is a unique museum. Fahrzeugmuseum Marxzell is a privately owned museum devoted primarily to cars. It contains more than 350 motorized vehicles ranging from cars and motorcycles to tractors and fire engines. On top of that, it also contains a surprising number of artifacts and displays related to local history.
Europabad
If there's one thing Germans are known for, it's their love of waterparks. And Europabad in Karlsruhe is a fun one. It has multiple slides, some of which are themed for snakes and tarantulas for some reason. In true German fashion, there are also several saunas. Most are traditional but one is Excalibur themed. It seems like putting a hat on a hat but it's hard not to appreciate the wackiness.
Alter Schlachthof
If you've ever wanted to spend time in an old slaughterhouse, Alter Schlachthof is the place to be in Karlsruhe. The municipal slaughterhouse was quite literally located here until 2006. Now, it's a creative space with restaurants, bars, offices, and more. In case you're worried, the slaughterhouse didn't close. It just moved locations to a different part of town.