If you do only one tour during your visit, I suggest the free three-hour "Alternative Berlin" tour run by this company. It is mostly walking, with a subway and train here and there. (Make sure to follow your young, fast, walking tour guide closely; several slow people were lost along the way.) This particular tour dabbles in Berlin's history but mostly covers the abundance of graffiti art that blankets this city—plus you visit the largest and most famous art squall. Since it's free, it mostly attracts the younger international crowd. This outfit also offers several more comprehensive (and paid) tours should you want to go in-depth.