I have only passed by Snäck on a bike trip but this hotel by the beach some 3 kilometres north of Visby is known all over Sweden for being a nightlife hub and I have to say it sounds quite tempting to party and then hit the beach with your friends. Moreover, there is a camp site next to it so you need not even get back to Visby itself if you stay there.
Dress Code: Proper but relaxed.
Visby is one of Sweden's great nightlife cities summertime and Stora Torget, the main square, is somewhat of a hub in the Visby night with Gutekällaren and several other places, especially on its western side. You need only decide which music sounds like your style when listening outside, or where the queues seem negotiable on a summer's night.
Dress Code: Decent but still relaxed, especially warm summer nights.