If it were me, I would prioritize visiting the mosaics in Ravenna if possible. However, it sounds like you may be looking to fly into Bologna, spend just one night (?) and then take a train to Ravenna the next day. If that is correct, then you would have at most a couple hours to sightsee in Bologna and have a good dinner. Bologna is known for food; reservations are advised. There are also a number of food tours. For sightseeing, depending on what time you arrive in Bologna, you might be able to do the walking tour offered by the visitor center -- check the Bologna Welcome website. If not, you could walk around Piazza Maggiore and the nearby historic center. If there is enough time, I would recommend the Santo Stefano complex of churches, parts of which go back to the 5th century. The Bologna Welcome tourist office in Piazza Maggiore has a handout with the opening and closing time for a long list of museums and churches.