If anyone knows Berlin and surrounding areas, it's Fat Tire Bike Tours. They have been conducting tours of the city for years, and offer not just a sightseeing service but real in-depth information, insight and anecdotes - and one of the keys to their success has been that bike tours are all they do; no energy is wasted on side-enterprises.
They've thought of all the little details too - such as accepting credit card bookings with no charge, providing gloves and hot tea on cold days (and it can get very cold in a Berlin winter), luggage storage, free wi-fi and internet terminals, and more... and they're so sure that you're going to get a lot out of your tour that they are more than happy to take payment at the end, when customers are satisfied.
There's nothing about this operation that has been overlooked. A booking includes bike (serviced regularly by mechanics), insurance, helmets, bikes for children and customers with special needs, and of course the services of native English-speaking guides.