Biking through the streets of Amsterdam is a must. It is such a great way to explore the city and how all the locals get from place to place.
I rented bikes from a place called Hotel Scooter--they rent both Vespas and Bikes. The service here is great and the bikes are in tip top condition + come with a lock.
NOTE: The photo above is not one of the bikes that Hotel Scooter rents--this was just the best bike photo I had ;).