July and August are popular—and busy. If you want to see the tulips, come mid-April to early May. You’ll find better deals and quieter crowds October to March, with the exception of New Year’s Eve. This is a unique and riotous celebration in Amsterdam, and well worth experiencing.
Amsterdam is compact, so walking or renting bikes to make use of the great bike lanes is easy. Yet public transport (trams, metros, and buses) also is excellent. Buy a chip card at a ticket vending machine or counter, or ask the driver for a tourist pass (there are one- to seven-day cards starting at $8).
August is the hottest month, with an average high of 63°F (17°C), and the coldest is January, with an average high of 37°F (17°C).
Some attractions (especially the Van Gogh Museum and Anne Frank House) get very busy, especially in summer, so booking a ticket and time to visit in advance. The city is generally safe, but pickpockets operate widely; be careful on the tram and at the market.