There are several bicycle rentals in Haarlem. One is at the railway station, another one at a few minutes' walk.
It's good to know that you can take bicycles on the train (not during rush hour). A bicycle ticket costs around EUR 6 and is valid all day. Take the train from Haarlem to one railway station, and let your trip end at another one. It gives you maximum flexibility.
The area north of Haarlem - between Alkmaar and Purmerend - is the real postcard Holland, including the windmills.
A popular area is just north of Amsterdam, with villages and towns like Broek in Waterland and Monnickendam.
Not very well known is the area south of Amsterdam, between Abcoude, Uithoorn and Ouderkerk. You could also have a trip from Abcoude to Naarden, via Weesp and Muiden.
The small road along the mid-17th-century Leidsevaart canal brings you in Leiden.
The immensely detailed OpenStreetMap shows you all the bicycle paths, either a separate path along roads or bicycle-only paths.