I'll be on vacation in northern Europe for the next two weeks, and I'll probably be looking to play poker 2 or 3 nights while I'm there. Never been to this part of Europe before, so I have no familiarity with the casino options there.

I'll be in these four cities for about 3 days each:
- Frankfurt
- Berlin
- Copenhagen
- Stockholm

Any recommendations for low-to-midstakes NLHE, LHE, or LO8 games in any or all of these cities?

Message me when you are in Copenhagen. It might well coincide with one of the days I am going to either a cosy casino in DK or the one in Malmö, Sweden. You can tag along.

No poker in Frankfurt, next casino is Spielbank Wiesbaden.
Berlin has three branches of Spielbank Berlin. Big, fancy one at Potsdamer Platz and smaller ones at Alexanderplatz and Hasenheide.

Good to know to avoid disappointment: Casinos in Germany charge a small entry fee, drinks aren't cheap and rake is significantly higher than in the US, capped at 20€ at 2/5. Not totally sure about 2/2 which is the lowest limit in Wiesbaden and at Potsdamer Platz.

Germany is not northern Europe. For the idiots, you will usually find that the level of play is better than at similar stakes in the US, just as a warning.
Berlin has a step structure that roughly equates to 5% capped at 20, same at 2/2 and 2/4 (no 2/5). 5/5 has cap 30. Tough to beat.
Actually there is also Spielbank Bad Homburg which is closer to Frankfurt but has only poker a few nights a week, and I think mostly micro donkaments. I've never been there.
Wiesbaden is a grinder-heavy place because it has very low rake but 1.5% mandatory "tip" (uncapped). I don't think madlex is correct here, unless there was some recent change the lowest game is 2/4 €. Games only run evenings/nights like Berlin and note dresscode (not very strict but you won't get in with too shabby/out of line stuff)