I'm pretty sure that Zoologischer Garten, on the X9 express bus, as well as Berlin
Hauptbahnhof and Alexanderplatz, both on the TXL express bus, have lockers. Which is the best option probably depends on where the drinks are. (E.g., Alexanderplatz is farthest from Tegel, but if we're meeting in Prenzlauer Berg, it may be most efficient to take the TXL bus all the way to Alex, ditch your stuff, and take the U2 to drinks.)
http://www.s-bahn-berlin.de/fahrplanauskun...en&modul=m1 is useful for sorting out the transit connections to an address.