My longest stay in any hotel was in the Hyatt Regency Mainz, and it is wonderful. With the exception of the Christmas holidays, I lived there from June 1999 through June 2000, and loved every minute of it. Definitely get a Rhineview room. The shower and bathroom alone are worth it! If you like contemporary design, all of the Hyatts in Germany are great.

The restaurant in Mainz is wonderful though--the kitchen is open so you can see everything going on, the buffet breakfast with anything made to order can't be beat, and you have to have the rack of lamb for dinner one night if you fancy that kind of thing--I had it a couple dozen times, and they change the recipe slightly every so often ;-)

The RC is always well stocked with beverages, although I will concede that the evening munchies aren't always the best, and the breakfast doesn't compare to downstairs, of course.

You can make it down to the Heidelberg castle in less than an hour, and to trendy Wiesbaden in ten minutes. The market in downtown Mainz--a five minute walk from the Hyatt--is great as well. The restaurants surrounding the hotel can keep you busy for a good month. Be sure to stroll around and then ask for recommendations.

I'll let others who know it better cheer for Hamburg, as it is wonderful as well, but Mainz is like a second home for me as of late!