If you have a spare day during your stay in Gothenburg, an excellent day trip, one that the whole family will enjoy, is a boat trip out along the archipelago (the grouping of many small islands that surrounds the city).  Trips out to many of the islands, or just in and around them, are available via public transport on Gothenburg's ferry system.  Public transport cards that can be used for the city's trams and buses are also valid on the ferries.

The boat trip out along the Gothenburg Archipelago is breathtaking, one of the world's most beautiful scenes.  If you choose to get off of the ferry at one of the islands, you will also have the opportunity to fish, swim, or rent a small boat.  The islands of the archipelago which are inhabited all have their own small villages and are free from any car traffic.  Ferry operators or hotel personnel can direct you to the best spots.   

During the summer months, the ferries make several trips out to the island each day.  All of Gothenburg's tourist information kiosks and hotels have schedules.