Gavnø Castle on the island of Gavnø has a large, beautiful park with seasonal flower displays. A magnificent display of spring bulbs can be seen during the month of May. The castle has an extensive collection of paintings. In addition to the castle and grounds, there is a church, a live, tropical butterfly collection, and a collection of historic firetrucks. It is open annually from mid-April until the end of August.
The boat Friheden ("Freedom") runs regular, round-trip service from May–August. The 2½ hour trip starts in the center of Næstved, and goes through the Næstved Canal, to the Gavnø Castle, and out on Karrebæk Fjord to the town of Karrebæksminde before returning to Næstved. It is possible to disembark at Gavnø and at Karrebæksminde.