Personally I believe the best way to tour Amsterdam is by bicycle. Transportation is fast paced…read morehere...one must always be on the lookout for bikes, autos, trams and obviously pedestrians. The city seems to encourage cycling for transport and has made it quite convenient for a bicyclist to maneuver around town. Bicycle lanes line all major roads and places to lock up are plentiful!
Word of warning which comes from my personal experience: take extra care to be sure your bike is properly chained when leaving it to explore by foot. I have experienced the misfortune of returning to where I parked and locked (or so I thought) my bike and discovering my (borrowed) bike had disappeared.
Holland Rent A Bike is the best and now only place I will go for rental when in town. The bikes are impeccably maintained, granted these are older style cycles, but I've never had to return during a rental period requiring repair. They provide a heavy duty lock and all bikes have a mounted back wheel lock. Best bonus: their rentals are not obvious tourist targets in that they are not brightly colored, covered with ads of the shop in which they are from.
The staff is friendly and will make sure you ride out on cycle fit to your size. The price is more than reasonable and if you're renting for an extended period of time, you may even receive a discount.
If you want to get around A'dam in style, Holland Rent a Bike will hook you up!