The national museum is in a beautiful wooden structure and worth visiting. Cambodia has had much Indian influence, for example the Wats of the Angkor and Hindu. So there were many an image of Shiva and Ganesh and other Hindu gods in the muesum. I have noticed that in each of Thailand, Laos and Cambodia's national museums they have shown maps displaying their largest empire at any point in time which is sometime in the distant past. The Khmer empire covered most of Laos up to Burma and most of north- eastern Thailand when it was its most powerful.
The Royal Palace is another place worth visiting. There is a pagoda with a whole floor of silver tiles. There is also a French building right in the middle of the palace grounds which looks a bit odd. Wat Phnom is pretty good because there are a lot of monkeys around which are amusing to observe. You can buy them lotus seed heads and bananas for them to eat- at least it's pretty healthy.