Whatever it used to be, the Brandenburg Gate is now a symbol of peace and openness. It is a great place for a meet up as well - so wide open that you are bound to see your rendezvous partner (just in case, there is also Starbucks for free wifi). As you come out of the metro (S-bahn Branderburg Tor) you can find park benches and places to park your bike. Learn about the history beforehand, it will make standing there a much richer experience!