Bologna for 2 days Hotel?

We want to spend 2 days in Bologna in September before joining tour in Padua, which area is best to stay in, does it have a specific historic centre? We want to be able to walk in interesting areas from hotel we would like a hotel with charm, we have like the RS hotels we have used on tours to give you some idea, location is important, and a rooftop area bar or viewing place is always enjoyable, not wanting big chains that could be anywhere, hope this isnt too confusing