We will be celebrating my Dad's birthday this coming Saturday in Berlin and were wondering if anyone had recommendations for a restaurant in the evening. Ideally it should be in Mitte and not too posh. We were thinking of Cafe Adler but are not sure how extensive the menu is in the evening. Any help would be greatly appreciated.

It's hard to reply if you don't give us a price range and the kind of cuisine. One place I go for special occasions is Restaurant Maxwell, Bergstrasse 22 in Mitte, where main courses run from 15 to 20 euros. They have an excellent chef and impeccable wine list and it's a beautiful place in an historic brewery. http://www.mxwl.de/maxwell.htm
Or for top-of-the-line Thai, Kamala on Oranienburger Strasse (go upstairs for a great view of the golden dome of the Old Synagogue) or its more beautiful cousin, Mao Thai on Wörther Str. 30 in Prenzlauer Berg. Or for more down-home classic Old Berlin pig's knuckle and sauerkraut, Zur letzten Instanz at Waisenstr. 14-16, supposedly the oldest restaurant in Berlin. http://www.zurletzteninstanz.de/a_sprache_eng/index2.htm
Those are only a few I could name. The Café Adler is AFAIK a café for coffee and cake and hanging out, not actually a restaurant for a birthday dinner.