The canal tour boat has several stops near the most popular sights, where you can get off and explore and then get back on and float to the next sight. The major stops are Anne Frank's House, the museums, the Flower Market, and the Red Light District. Whether you get off or not, it is worth the trip to see the sights pointed out. Be sure to get an early start, as the museums and the other sights may take some time.
The Rijksmuseum has the most famous collection of old Dutch Masters and is must-see. At the same canal boat stop is the Van Gogh Museum, another important sight.
Even if you are not a gardener, everyone enjoys the Flower Market. It is a good place to find inexpensive souvenirs and a hearty lunch of Dutch sandwiches and beer.
If you are after a little adventure, you will want to stop at the Red Light District. Watch your wallet and passport. Also, photographing is frowned on.
The "brown" cafes are popular with the young tourists, based on the belief the marijuana sales are legal. They are not, but are generally ignored. Lately, there have been raids because of stronger drugs being sold.
Amsterdam is notorious for the aboveboard gay life, and it is pretty obvious. The city attracts gays from all over the world and alternative lifestyles are quite visible.