For a last minute weekend trip to Berlin, I found an excellent bicycle tour group, Berlin on Bike.
They offer two regular tours in english almost every day of the week for very reasonable prices (4 hour tour= €19 including bicycle rental). They also have lists of specialty subject tours and the possibility of custom group tours.
However, the regular Berlin Wall tour was great and can be done on the spur of the moment (I did make a reservation the day before).
Our group consisted of 13 people from various places, about the right size for riding in a city. I volunteered to follow the group in a bright orange vest until a kind Dutch fellow took over to wear his national color.
A few participants had their own bicycles (Poland) and the rest of us rented bicycles (Denmark, Netherlands, Germany, Mexico, Colombia). It was a nice, slow, informative tour that took us to some non-touristy areas that I didn't know about. Our guide, Joachim, was very good and told us a lot of facts and stories during the ride.
I heartily recommend them for a great way to see Berlin!