A couple of days ago I took my parents to one of my favorite summer restaurants: Fjäderholmarnas Krog. The restaurant is located in an island group called Fjäderholmarna, just outside the city in the beginning of the archipeligo. It takes a 30 minute boat ride to get there and there are regular boats going from both Slussen (10 minute walk from the Rival Hotel) and Nybroplan... and the beautiful boat trip is part of the charm as it takes you through Stockholm harbour past Nacka and the island of Djurgården. The food is great with the menu emphasizing fresh seafood, but there are also meat and vegetarian dishes on the menu. I had the duck cooked three ways (smoked, grilled and confit) and it was delicious. It is the atmosphere, however, which lures people here again and again. It is almost hypnotic with crying seagulls and water lapping against the pier. The islands are located right at the entrance to the harbour and from your table you can watch the boats go by... everything from tiny sailing boats to large cruise ships.
Besides the restaurant, there is lots to do on this beautiful island. Many Stockholmers come out for the day to swim and have a picnic. There are quite a few handicraft stores, featuring glass-blowing and textile work. There are smaller cafés, other restaurants and a theater as well. I would recommend coming out to the islands an hour or so before your reservation just to wander about and soak in the atmosphere.