A few years ago I did a short cruise of the Baltic Sea, and I chose a particular cruise because it included Visby. I did spend a few hours walking through the historic streets, but unfortunately it was raining so heavily and the wind was blowing so strongly that it really was no fun at all. I did see the museum, which was definitely very interesting, but the rest I could hardly see (and it was the beginning of July!). Fortunately, on my second visit (by ferry from Nynäshamn), the weather was much better (also in July). So I could finally explore all the parts of Visby I had missed the first time around, in particular the very impressive city walls. If you are lucky with the weather, Visby is one of the finest historic cities in Northern Europe.