For one week in early August, Visby drops the 21st century and goes full medieval. Jousts, markets, theatre, music, fire shows, public sword-fights, costumed reenactments of the 1361 sack of the city — the entire walled town becomes one big street festival. It is unironic in a way that surprises first-time visitors and joyful in a way that surprises everybody.
Book accommodation months ahead. The dates change each year — usually the first or second week of August — and many events are free, but the staged tournament inside the walls charges admission. Even if you avoid the official programme, the atmosphere in the streets and ruins is worth being there for.