The most important temple of Phnom Penh and maybe even Cambodia is Wat Ounalom. It’s the seat of the Mohanikay order of Cambodia. An order affiliated to Theravada Buddhism. The temple was built in 1443 and got same as all religious structures or buildings destroyed by the Khmer Rouge. That happened in the late 70’s. Now all is restored. On the compound you will find statues and stupas and even a sort gong with the flags of the ASEAN nations. The location of this temple is along the river and near the palace and has free entrance as all temples have.
The smaller stupas at Wat Ounalom belong to individuals who passed away. Cremated at the temple compound, their ashes in the stupa and a picture on it will bring the person who passed away a good reinarnation. In the main stupa of Wat Ounalom is believed to be an eyebrow hair of Buddha. It is allowed to take pictures in the temple up the stairs. Leave your shoes outside and walk in.
Often you can get yourself blessed by a monk. You kneel in front of the monk with your hands in front of your chest to each other. He will do some prayers or chants while throwing some water drops on you. You can get a cord around your wrist and give him a small donation for bringing you fortune.
The national Museum of Cambodia is located in Phnom Penh. It’s a center of historical and archeological value. It has to be said that it’s hardly any different from all other museums in Cambodia despite the thousands of artifacts.
You can find the exact same items and topics at the Angkor museum in Siem Reap for example. It’s all about the Khmer empire time, old tools that were used at the time. Old materials and different types of stone, statues and more of that.
More info here https://www.cambodiamuseum.info/en_information_visitors.html
Arts & Ceramics
Khmer arts, ceramics, sculptures, bronzes and statues. With over 14000 items it is large but it is also more of the same. The museum was opened in 1920 and renovated in the late 60’s. During the genocide of the Khmer Rouge it was closed and many items damaged or stolen. In ’79 the museum opened again. When you’re interested in archeology and this kind of history, the museum is surely a place you should visit and you can spend easily 2 hours or more inside with reading all the associated information by the items.
For other people this museum is nothing more than a collection of stones and statues that are all similar and that you can stare at for hours without seeing anything special. Those people could finish the whole museum in 20 minutes and for them the $10 entry ticket is rather expensive. This price is without audio or guide. Take also into account that taking pictures is at most places prohibited just as wearing a cap.