I've been to Berlin a couple of times. Very cool city.
Personally I thought the TV tower was a bit too expensive and not all that much to see. The city is much more impressive from the streets than from above.
But if you go there you can walk down to Alexanderplatz, and on the way look for decorated houses with courtyards, some of them are beautifull and filled with very impressive decorations.
"Behind" Alexanderplatz is a huge park that has a monument to Sovjet soliders that I can recomend taking a look at.
At that point you could go back to Alexanderplatz and take a tram to Checkpoint Charlie I think (maybe you can walk, but I've always gone by tram). At CC just beware of all the "official museums" and stuff, nothing rally worth checking out. But the area itself is nice.
When I last went there we actually asked a man in the tram for directions to CC, turns out he lived right there dring WWII, and he actually went with us there and told us how it was (very emotional, he started crying several times and told us lots of horrible stories).
From CC I think you can walk to a part of the Berlin Wall that's still intact, and near that poin is a large tram station that should be able to take you anywhere.
As you walk around town, be sure to stop for a drink as often as you can. There are a lot of really awesome bars and cafés around town, especially in the backstreets where not as many tourists are. You should also avoid any major street restaurants as they're quite expensive and don't have as tasty food as the backstreets. Altthough this is just common tourist sense.
Uhm, other than that, I can't really remember what there is to visit. I rarely aim to visit all the sights. Also try not to stay out of east Berlin (some do, becase they think it's worse or something). East Berlin has a huge indie/undergroud/arts scene and there are a lot of shops, cafés and bars that really stand out.
I hope that was of some help at least, I'm trying to remember what I've seen and done there, but I'm terrible with names.