Bologna is one of the best kept secrets in Italy, both for the food and the (underrated) sightseeing ops. I would recommend doing some reading on the various sights as many of them have interesting back stories. The Duomo in particular has an interesting history. For me the best food in Italy is in Bologna (As Roberto said, Italians call it "Bologna la Grassa", Bologna the Fat, for a reason). I have a close frined who lives outside the city and the first time she showed me around she took me to a place called Gelateria Gianni and said it has a repuation as the best gelato in Italy. I've had gelato all over the country and I'm inclined to agree, it was a cut above any I've had anywhere. Everybody I've taken there since has agreed. You have to go to the main one on Via Montegrappa though, not the new one near the Two Towers. Speaking of the Two Towers, try to go up in the one that's open; it's a long climb but the views from up there are nothing short of phenomenal.
Also, check out the Quadrilatero section just off the main piazza, it's a maze of little streets in the center of the historic zone dating back to the Roman times and it's chock full of food shops and stands. I could easily spend all day meandering through there sampling local goodies.
Cripes, you've got me all excited about Bologna now, I may have to head down there this weekend, lol...