One fun activity that me and my friends like is renting kayak (for example at Kajakkompaniet Stockholm AB to have a great starting point) and explore Lake Mälaren – or if you are more adventurous than me, why not go further and pass Gamla Stan and Djurgården?
This kayak company is located just over 5 minutes on foot from the subway station Kristineberg (10 minutes with the green line going west from the central station). You can choose between the options of going by yourself, or if you are less experienced you might prefer to follow a guided tour.
I strongly recommend booking in advance, and to check the weather forecast. Personally, I am a fan of paddling towards the direction of Solna, away from the city, and following the coastline, but there are endless options. I enjoy the stillness of the water, the richness of the wildlife, especially birds and one other advantage is the different perspective that you get of the city when you see it from the water. There are many options where you can stop for a snack in a café or restaurant, but naturally you can also bring a small picnic with you and stop wherever it is convenient. Bear in mind when bringing snacks to pack them WELL as they run a huge risk of getting wet in the kayak.