If you only do 1 tour during your visit, I suggest the free 3 hour "Alternative Berlin" tour run by this company. It is mostly walking with a subway and train here and there (TIP: make sure to follow your young and 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 plethora 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 more in depth.