Roskilde Fjord stretches north for about 40 kilometres from the town to Isefjord, a long, sheltered inlet that was the waterway the Viking ships were sunk to defend. Today it is one of the most popular sailing fjords in Denmark — calm, relatively shallow, and dotted with small wooded islands. You can rent kayaks and small sailboats from the Viking Ship Museum's harbour in summer.
The fjord path running north from the museum along the waterfront is good for an evening walk. In calm weather the reflections of the town and cathedral in the still water are worth the fifteen-minute stroll from the centre.