Our son and I are in Berlin in June staying at the Holiday Inn Express in Berlin Centre (Stresemann St) . Where is a good place to eat - somewhat informal and not too expensive?
Schwarzwaldstube in Tucholskystrasse (near Oranienburger Strasse S-Bahn station). Get there early-ish before it fills up.
If you are looking for a German restaurant, I have greatly enjoyed more than one meal at Augustiner Am Gendarmenmarkt. Great beer, and everything I've had there is very, very good, but in an informal setting, and pretty reasonable prices also. Has a nice vibe also...and afterward, take a short stroll through the Gendarmenmarkt, and enjoy the architecture of the French and German cathedrals, and the Konzerthaus.
Some of the best food we ate in Berlin was the street food. Cheap and yummy.
There's turkish Doners, and the one that was recommended by a local was this Mustafas Germusedoner. http://www.yelp.com/biz/mustafas-gem%C3%BCsed%C3%B6ner-berlin Its opened late, and you can't miss the line.
We also had to eat CurryWurst, and you can find stands all over. The one we had was near the Zoo at Curry36
Both places are accessible by metro.