I love Berlin and could live there. I have friends in the city and have been many times.
Spring is good, summer lovely but you might get a heatwave. It’s still quite a northern city. Just avoid the chilly winter!
I can’t recommend more highly doing a walking history tour. We did this one which took us all around the centre in 3.5 hours, covering the founding of Berlin to the modern day. I found it fascinating: from Napoleon to the Weimar Republic, the rise of the Third Reich, WWII, defeat of the Nazis, the slicing up of Berlin by the allies, Soviet rule, communist Berlin, the fall of the Wall, the reunification of the city, the active memorialising of the atrocities by the people of Berlin as part of “never again”, the deliberately transparent parliament building that’s open to all.
Standing on the spots where the Russians liberated Berlin, outside the university where the Nazis burned the books as students cheered them on, the place where Hitler killed himself, sites where people made desperate and courageous escapes over the Berlin Wall… it’s the history of our continent in one city.
We did it with our kids aged 14 and 10 and even they enjoyed it (I did step away with my ten year old for a couple of the less appropriate parts - Hitler’s bunker) but there were teens on the tour too. We used this company: Berlin History Tour
The tour provided breathing spots including a couple of bakeries to take it all in, but afterwards we wound up at mt favourite Berlin summer spot - the Prater Beer Garden in Tiergarten (Berlin’s answer to Central Park but twice the size - hire bikes!) and wound away the afternoon drinking, eating and boating in dappled sunlight and reflecting on all we’d heard.
I also recommend the DDR Museum, the Spy Museum, the East Side Gallery, clubbing in the old bombed out warehouses, and if you have time to go to the suburbs, the 1936 Olympic Stadium - more chilling yet uplifting history. Berlin is surrounded by wonderful countryside especially lakes for swimming in so if you do go in the summer you could include a day there to cool off with the locals. Enjoy!