If you can find a cheap flight I would highly suggest Riga. It's by far one of my favorite cities in Europe and fairly cheap.
Staying closer to where you are, I've really enjoyed visiting Maastricht. The old town is incredibly charming and it has a wonderful jazz scene due to the music conservatory. There are a couple cool churches converted into modern amenities, my personal favorite being this bookstore. Eben-Emael is also pretty interesting to check out if you're into WWII history.
I would highly suggest against going to Stuttgart, but if for some reason you find yourself coming this way you are more than welcome to crash on my couch.