Last week, when we walked around Djurgårds Canal, we stopped at Rosendal's Garden (Rosendals trädgård) for a light lunch. Located on the island of Djurgården, behind Skansen, Rosendal is an open garden with orchards, vinyards and roses as well as greenhouses. They have a great café where you can get a lighter lunch, salads, sandwiches or cakes and cookies. All of the food is made using organic produce and the vegetables come from their garden and the bread is fresh from their bakery. One nice detail is that you can either eat indoors, in the garden or take your tray out to the orchard and use the picnic tables. There is also a cute shop where you can buy selected organic foods, bread from the bakery, oils & vinaegers as well as ceramics. Many tourist sites, like Skansen and the Nordic and Vasa Museums, are nearby. To get there just walk along the north coast of the island from the Nordic Museum and follow the signs for Rosendal. Their summer opening hours are from 11am to 5pm weekdays and 11am to 6pm on the weekend.
If you are more in the mood for a sit-down lunch then I would suggest Rosendals Wärdshus or Sturegård, which are both located in the vicinity of the gardens and offer both indoor and outdoor seating.