Stureplan is where all the high end nightclubs are and around that area is also a good place for daygame.
For daygame i would focus on Drottninggatan, sergelgatan, hamngatan, gallerian(mall), NK(high end mall), kungsträdgården(park), then around stureplan which is basically around Östermalmstorg T-bana(metro station). Here is where the high end girls are during the day aswell, hotter high end girls with expensive clothing.
Drew a route on a map you could walk to get started.
There will also be many girls around Götgatsbacken, which is a "hill"/road between the metro stations Slussen and Medborgarplatsen.
At medborgarplatsen there are many outdoor bars at the big square you should check out. Lurpassgame is nice here = sitting in the sun with a beer waiting for prey to walk past(there will be plenty if good weather).
Walking strandvägen towards Djurgården with the water to the right and Swedens most expensive apartments to the left is a nice walk, and also girls to open.
Someone mentioned Solidaritet, i havent been there, im not big on the nightclubs but i know a lot of guys like to game here, i got the impression its on the lower end of the high end clubs if that makes sense haha.
Overall in Stockholm you will find plenty of girls. If it is good weather and you have a car some places to swim could be worth checking out.
Dont get stuck around Stureplan, there are more places to see.
In gamla stan you will find thousands of tourists, if your lucky you will find solo travelers here with no idea on what to do next.
Edit: To answer you question where to stay: Close to Östermalmstorg for best night game logistics.