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!
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.
What looks like a plant nursery from the outside, is both, a plant nursery housing two rescued Amazonian parrots and a hidden cafe serving up some of the loveliest cakes and cuppas in all of Berlin. Brace yourself fir occasional screeching parrots flying around though that only adds to the charm of Blumen cafe.
Just an amazing cup of cappuccino. Try the Franzbrötchen with it for a basic breakfast. It's a place for someone who appreciates good coffee done with extreme care.
A very nice little second-hand bookstore, including a shelve with international books in French, English, and Spanish too. Always nice people to meet and a great selection of art and cook books, for affordable prices.