There's nothing more revigorating during the dark winter days than spending a couple of hours in a sauna. Germans have mastered the art of sauna and you can find amazing pieces of architecture in the public swimming pools (Stadtbad). I find Stadtbad Neukölln very peculiar in its neoclassic architecture and it's definitely a wallet-friendly option to enjoy some sauna time there. Be prepared to be naked in front of man and woman!
Just until recently this place used to be one big old Aphoteke (Pharmacy). They have kept the Apotheke's dark wood Art Deco charm to use in benefit of great coffee and fantastic brunch (try the french toast and you'll see I'm not lying).
The Tempelhof Airport is now closed and they have transformed the landing field into a public park area. As soon as you go there you notice that usually you don't get to see that much sky in the city so entering this area for the first time is quite overwhelming. I especially love the old softball fields that have been built during the American occupation.
This chapel at Dorotheenstädtischer cemetery hosts a light installation by THE James Turrell at sunset on specific days. It begins with a 30minute intro and needs a booking to visit.