Berlin's a great city, you can't really go too far wrong. Most of the pubs are welcoming and well priced. Ideal for a short lads' summer trip.*
The Brandenburg Gate is worth seeing - we were lucky enough to be there at the time Germany were playing the Netherlands, so there was a big screen up and thousands of expectant German fans. Also the Berlin Wall, but you probably knew about that anyway. Plenty of history to be seen as well as some funky modern art/vandalism.
There's a tower in the centre from which you can get great views of the city, and again that's not too pricey, so I'd recommend that. Can't remember the name for the life of me though.
The Olympic Stadium is a strange one, the team hasn't exactly been incredibly successful so there's not hordes of trophies like with other stadium tours, but it's the same ground that was built for the Nazis' Olympics so it's a very interesting combination of their desire to create a legacy, and the modern-day use of that legacy (obviously all the NSDAP stuff is at a minimum, but they still use the ground - they are Germany after all, anything else would be
very inefficient!).
Oh, and make sure you get yourself some Currywurst. There are stands all throughout the city centre serving it, and you generally get it in a little box, with optional sides of fries, potato salad, and a big bottle of
water beer! (which you can drink much like someone would slurp down a coke bought from a shop on the streets of the UK, they're quite happy for you wander about with a bottle so long as you're not a massive twat). And again, the pricing is very reasonable indeed. A quick google provides this list
this top ten which might be of use - when we went, our hostel was about 15 mins walk away from the supposed "best Currywurst stall in Berlin!" I can't recall its name or if it was one of these on the list, I'd wager it probably was, but it was worth the walk - and all for only a few quid, large beer included.
*The trip will be the short part, I am not implying that you are somehow bereft of height
EDITED for beer. Mmm, beer.