The Rijksmuseum and Van Gogh Museum both deserve a half day each, and booking timed tickets online ahead of time avoids the worst of the lines. The Anne Frank House is genuinely moving and also requires advance booking — tickets go fast. A canal cruise is touristy but still a nice way to see the city from the water, especially in the evening when the buildings light up.
The Jordaan neighbourhood rewards slow wandering with no particular destination in mind, and the Albert Cuyp Market in De Pijp is worth a visit for cheap eats and a more local crowd.