The Opstapper is a minibus, a small Mercedes affair. Yes some ski equipment would probably fit if there are not to many passengers on board. There often are not, such that the Opstapper has been threatened with withdrawal several times. The Service runs from outside CS, but outside the Connexxions bus station by the VICTORIA HOTEL. It runs down the Haarlemmer Houttuinen to the Prinsengracht and runs its length effectively to the Stopera. It is fairly frequent. But as the previous entries say check the GVB site
www.gvb.nl