2 weeks in Copenhagen, where else to go?

My partner and I (both men in our early 40s, no kids) are going to Copenhagen for the first time. We'll be there for about 2 weeks in late June/early July. We'd like to add on a 4-5 day excursion to another city, ideally on a weekend so we can experience some night life. So far we are considering either Berlin, Stockholm, or maybe Amsterdam but I was curious if any travelers here might have some advice!