There is so many nice campingspots in Mälaren. Here are a map wit suggestions of a few islands sutible for camping. But there is nice camping spots on almost every island. Please see the map.
MAP IS UNDER CONSTRUCTION
East coast (Stockholm, Mälaren and Gotland)ToröMusköDalaröÅkersbergaMariefredGotland
West coast (Gothenburg and Bohuslän)Gothenburg/FiskebäckGothenburg/HisingenTjörn