Walk under the 666 arches that lead to the Santuario della Madonna di San Luca, with its lovely (but not extraordinary) views over Bologna. Starting at the Meloncello gateway is actually nearly the halfway point, at arch number 300-or-so. The walk, for an average healthy person, should take between 35-45 minutes. I'd recommend tackling this after lunch at the nearby Trattoria Meloncello (also listed here). Be aware that there are no shops at the Sanctuary, so take plenty of water (there are public washrooms at the top though, fortunately).