Boserup Forest is Roskilde's easy nature escape, a large woodland west of town with paths through beech forest and access toward the fjord landscape. It offers a calm counterweight to the cathedral, Viking ships, and festival-site associations.
The forest is best for walking, cycling, picnics, and a low-key half day outdoors. Come in spring for fresh green leaves or autumn for colour, and combine it with the fjord if you want a fuller route.