It might sound crazy to visit the same city twice in one year, but Hamburg is definitely worth it. After having been there this February, my boyfriend and I decided we wanted to spend some days there again this summer. And we did.
For those of you who prefer hot, sunny days and the beach it might not be the right place to spend the summer. But for all the others: the days are long and the city glows!
Walk around and keep your eyes open. For me, that's the best way to get a feeling of a city.
One that is particularly stunning is the old botanical garden, which is part of the Planten un Blomen park. It's perfect if you're looking for some silence. Although in the midst of the city, it's quiet and peaceful. There are chairs everywhere, so take a good book with you and just rest there for a while.
My third recommendation requires some good shoes: Take the S-Bahn to Blankenese, the wealthiest part of town, have a look around and then walk down to the Elbe. From there, you could walk all the way to the city center, but that might take you about 5 hours. We did about half and then took the bus back, which was enough for one day. If you're not up for walking, you can rent bikes, too.
However, whether in winter (when it's freezing) or in summer (when it's still not too hot), Hamburg is worth going to!