We should really do a faq on this. There are already a lot of suggestions under "Berlin im Mai" and "Going to Berlin in November."

A visit to the Museum Island is more or less obligatory, as is the Museum at Checkpoint Charlie. You can get a bus or boat tour with an English-speaking guide, if the language is a problem. Berlin is a city rich in museums and historic sites, your guide book can give you a long list. Some out of the way places on the edge of town:

Wannsee House: Go to Bahnhof Wannsee and take the 114 bus or simply walk west along the shore until you get there. The Wannsee House is where the Holocaust was planned, now it's a museum.

Teufelsberg. Start at Bahnhof Heerstrasse and walk or bike SW along the Teufelsseestrasse. Take one of the inviting footpaths west to the mountain or go all the way to the parking place and start your climb from there. This small mountain (120 m) was built out of rubble collected after the war. It provides an excellent view of the city and is a good place for picnics.

Sachsenhausen. Take the S1 to Oranienburg, then a cab to the concentration camp. (The walk is long and not very scenic.) You can get a guide and other literature at the entrance. Be sure to vist the museum and get yourself photographed at the "Arbeit macht frei" gate. Then go have a nice supper, because visiting a concentration camp can be depressing.