If you decide to visit Bologna as a daytrip, make sure to visit the Basilica di Santo Stefano and Piazza Santo Stefano. This was our favorite part of Bologna. The Basilica is Bologna's most unique religious site. It's a medieval labyrinth of 7 interlocking religious structures dating back to the 11th century. It is so unique and so interesting. And we preferred the smaller and more intimate Piazza Santo Stefano over the much larger Piazza Maggiore.
We did the opposite of you, however. We spent 3 nights in Bologna and visited Ravenna as a daytrip. I know that Rick prefers Ravenna but to us Bologna is more interesting.