You should narrow down your question/questions, but I'll answer as best I can.
You should definitely go to Sweden, since the country is at its best during the summer months.
For second-tier cities I'd actually look into going to an island called
Öland (biggest town Borgholm) during the middle of July. There are big dance events and parties there during those weeks. I haven't been myself, but friends (both natives and foreign) report that it's a good experience. Many girls go there with their social circles, but are open to new dalliances. It's usually far enough from their hometowns that they can slut it up. It's a fuckfest since Swedish people binge drink during vacation, we make the most of the few months there's actually sun here.
The best vibe to have is to have fun and be relaxed, it's okay to tone down the cockiness and back-and-forth a bit when seducing Swedish girls. As soon as you isolate be more direct, they usually reward boldness but be calibrated and mindful of your surroundings.
Look for the chance to fuck in an old medieval castle ruin on the island, if you go there you'll know it...
The dance events that I've heard of are not EDM, but more old-timey modern jive music (again, could also be other EDM events as well since I haven't been there myself).
In conclusion: Swedish girls + alcohol + sun + summer vibes = horny (just like everywhere else).
Natives usually leave the capital Stockholm and the 2nd biggest city Gothenburg during the vacation season, since tourists invade these cities in the summer months. The native girls go to smaller cities/towns like:
Smögen
Båstad (city with a cream of the crop feel, some bigger tennis competitions are played here)
Halmstad (awesome beach there called Tylösand).
Visby is a beautiful old town on another big island called
Gotland, which is packed with both native Swedes and tourists during summer.
Kalmar close to Öland has a big city party during summer
If I was a foreigner looking for a summer romance, these are the places I'd check out. Of course you should see Stockholm as well, but for 2nd tier cities these are a must. Bit of a ranty post but best of luck, have fun and enjoy!