Last sunday I went with my friend Thomas to the islands near Oslo. It is a nice trip, and the most amazing thing is that you can use the same ticket that you use for buses since the ferry is still part of the same transport system.
Don't imagine islands like caribbean islands, this is Oslo after all. These islands are small, cold and with a vegetation that is very different from palm trees and coconuts. But overall, they are very nice and very relaxing. I spotted a few places where snorkeling could be an option, so I need to get snorkeling equipment (I already checked and it costs around 2000NOK to have a decent equipment) and wait for the temperature to become a little warmer and then I'll go snorkeling and see what's out there.
The only problem in the islands is that it was forbidden to start fire except in two specific spots, so barbeques are out of the question. I hope I can go tomorrow again, if it's raining they may be very interesting.