Tour on the river
Under the city of Bologna lies a river (which also makes Bologna horribly humid and hot in summer). It's maybe hard to believe but in Via Piela you can see the window that proves there really is one. Better yet is to discover the subterranean Bologna, by boat or on foot. There's a whole maze of tunnels under the city that have lots of history. You can choose between several tracks but you have to call to reserve. Just go to the Tourist Information Office at Piazza Maggiore for more info because they're still working on their English web page.