Living in Germany, I've heard of Oktoberfest... Sounds like a great party, though if you want to go to the big spots (ie. Munich), you'd better book now - if there are still tables available. But just about anywhere is going to have some sort of Oktoberfest party going on, so even if you can't get in to the main tent, you should have no difficulty finding some place in Germany in which to experience a beer.
As far as where to go, that's a bit of a tough one to throw out to the boards - I mean, I've been to 16 of the 27 countries included on your pass, and could easily make a case for any and all of them, but it's possible that you wouldn't enjoy the things that I do...
Since you have plenty of time to figure out your plans, I'd get a couple of general travel guides from the library and do a bit of research to figure out the kinds of things you and your mates are interested in (sports? music? partying? history? art? ballet?) and work out if there's anything, besides Amsterdam, that you all agree are must see's (or that some of you agree on - there's nothing wrong with splitting up part way through and checking out different things) and then trying to work out some sort of itinerary that will take you to some of the places you want to see. Once you've got a broad idea of either where you want to go, or of what you want to do/see, delve into these boards and read some of the threads covering them, and then start asking more specific questions based on what you've discovered.
Travel is personal, so you'll end up getting lots of (often conflicting) opinions, but they'll likely all help you make your decisions and enjoy your own, personal, travel experience.