Every Friday night (except in rain and snow), the Vondelpark is the start of a 20km, two-hour-long mass skate through Amsterdam. It's open to anyone with reasonable skating proficiency (ie knowing how to brake!). Arrive at the meeting point, adjacent to the Vondelparkpaviljoen by 8pm (8.15pm in winter). Check the website for details of skate (and safety gear) rental outlets.