It depends how you want to spend your time in Bremen. If you are looking to take a city break, then you should spend two to three days in Bremen. If you are looking to visit the historic sites, then you should spend at least four to five days in Bremen. You can visit the old town, the museums, and the castle. Bremen has a lot to offer and you should definitely visit the Bremen Fairgrounds during your stay in this city. This will be one of the best things to do in Bremen!