Strömkajen is your gateway to the Stockholm Archipelago - from here you will find lots of boats leaving on various routes to the different islands. You can for example go on a daytrip to Grinda or Sandhamn from here and my favourite part is probably already the boatride along the scenic route! I usually bring some pastries from nearby Stora Bageriet with me (they are open early in the morning) and then go for a walk on the island - for example Grinda. Pro-tip: during off-season you can use your SL ticket if you have a 30 day or more ticket on their boats, but beware that most places on the islands are also closed.