Near Gothenburg is the beautiful southern archipelago. There's a resident population on most islands, so ferries run all year. Private cars are not allowed. Take tram 11 or 9 in summer to Saltholmen, and then one of the ferries to Styrsö, Köpstadsö or any of the other islands. Here you can swim in the ocean, buy ice cream and maybe eat at the restaurants on Brännö, Styrsö or Vrångö. Be warned that the tram is packed with people on sunny summer days, especially when the 9 doesn't run, and going by car isn't better since you are unlikely to find a parking place. The ferries, however, usually make extra tours such days.
Styröbolaget provide a folder (http://www.styrsobolaget....) with more information about the islands.