Grab your bike and go explore the surrounding of De Stad ("the City is the nickname of Groningen)! For a good start, go to the Hoornsemeer, a lake not far from Haren but still closer than you may think, it's approximately 20minutes by bike from the city center. I recommend going on a sunny day, maybe in autumn or spring, it's really with it! Once there, you can enjoy a long (or short, as you want) walk around the lake, admire the clear water and the green forest, play in the kids playground (the swings are fun and you have a great view from here). It's a really good place to get some fresh air nearby the city!