Wat Langka is a gorgeous (and might I add, clean) pagoda situated close to the Independence Monument in Phnom Penh. The pagoda was used as a storehouse during the Khmer Rouge era and therefore managed to avoid total destruction. It is quiet and peaceful, and the stupas are kept in a great condition. While you are there you can take a walk over to the Independence Monument and the Statue of King Father Norodom Sihanouk.
Entry is free, and you are unlikely to be approached by beggars or sellers, meaning you can take your time to stroll around and take in the history.