The Maritim is in a great location to the Altstadt and has comfortable rooms, nice amenities and one of the best breakfasts I had experiences at a hotel in this price range in Europe. Try to get a room on 7, better ventilation and balconies with nice views of the bahnhof, castle, etc. I would highly recommend this hotel.
Unless you have need for one, don't bother to rent car either. There is great public transportation from the airport.