If you ask a Stockholmer where to take a date, they will not say Gamla Stan and they will not say the Vasa Museum. They will name a small handful of places that have been their default for years, the kind of places that aren't trying very hard and don't need to. Here's the actual list.
Rörstrandsgatan, basically the whole street
Four blocks in Vasastan, between Karlbergsvägen and S:t Eriksplan, lined with restaurants and bars whose tables spill onto the pavement from May to September. Storstad is the anchor. Lemuria does Italian. Tranan, around the corner on Karlbergsvägen, has been the local bistro of choice for forty years and the basement bar is still where people end up on a Tuesday.
The Vasastan move is to start with a drink at Storstad's bar without booking, see who's around, and then drift to whichever table opens up. It is not a planned evening and that's the point.
Nytorget on a Sunday afternoon
Nytorget is the small park on Söder that locals treat as a living room from April through October. The cafes around it, especially Nytorget 6 and String, are a good Sunday move because they slide from coffee into wine without you having to announce the transition. The park itself fills with people in their twenties and thirties pretending to read books, and a walk through it on the way to dinner at Nytorget Urban Deli or Pelikan is essentially the standard Söder date template.
In winter, the same blocks work but you stay inside longer. Café String has couches.
Hornstull on a summer evening
The western end of Söder, around Hornstulls Strand, is where younger Stockholmers actually go. The Sunday market by the water runs from spring through early autumn, the small bars along the strand are cheaper than anything central, and you can walk along Liljeholmsbron afterward and watch the sunset hit the water. Liljeholmskajen across the bridge is newer and less interesting but the boardwalk between the two is lovely.
Babylon, the bar built into the Bio Rio cinema, is the spot to know. A film and a drink there is a complete evening.
Långholmen for the daytime version
The small island just west of Söder is where Stockholmers go on a first date that isn't ready to be an evening yet. There's a beach, a swimming spot, walking paths around the whole island in about forty minutes, and the old prison is now a hotel with a decent cafe. In summer you bring a blanket and a bottle of something. In winter you walk fast and end up at a bar in Hornstull.
This is the date you suggest when you're not sure yet but you want to find out.
Mälarpaviljongen, but only in summer
The floating bar in Rålambshovsparken on Kungsholmen is a Stockholm summer institution. It's gay-friendly, mixed, loud in a good way, and the deck floats on the water with the sun setting behind it. From May to September it is the answer to the question "where should we meet on a Wednesday after work." In winter it is closed and you forget it exists.
Hidden in plain sight: the coffee date that becomes dinner
The move locals actually use is the open-ended afternoon. You meet at a cafe, somewhere with stamina, like Cafe Pascal in Vasastan or Drop Coffee on Söder, and you let it run. If it's working, you don't book a restaurant, you walk somewhere and figure it out. If it isn't, you've spent an hour on coffee and you go home.
This sounds non-committal because it is. The Stockholm dating norm is to under-promise on the format. A coffee can become a walk, a walk can become a drink, a drink can become dinner. Nobody books a four-hour evening on a first date. The city is small enough that you can chain things together as you go.
What we don't do
For the record, the things tourists are told to do for romance, the dinner cruise around the archipelago, the Skansen evening, the carriage ride in Gamla Stan, are not things any local has ever done on a date. Skansen is for visiting relatives, the cruises are for company outings, and Gamla Stan after dark is mostly closed.
Go to Rörstrandsgatan instead. We'll be there too.