Agersø is one of the small islands around the island of Zealand. There is no bridge connecting the island with Zealand but there is a ferry from the small town of Stignæs just across the water. The ferry trip only takes 15 minutes so it is an easy trip out there. You need to buy a ticket for the ferry at the port to get onboard and the ticket is always a return ticket since they assume you want to get away from the island at some stage.
There are actually two different ferries at the harbor in Stignæs the one going to Agersø and another island called Omø which is a little bit further away from Zealand so this trip will take you a bit longer.
Both the two small islands has a bit over a 100 people living on them and a fairly big tourism industry in the form of a harbor with lots of space for visiting boats and some summer cottages for rent.
The island of Agersø has many daily departures. During the day there is a departure an hour on the hour from Stigsnæs and half pass the hour from Agersø harbor. At night time there is only a departure every second hour so better be on time for the evening departures.