Phnom Penh International is about eight kilometres from the centre, which makes it one of the easier Southeast Asian capitals to see something of on a layover. With six hours you can do one serious thing; with ten you can do two and eat properly. Travellers who compare Phnom Penh flights on Traveloka should work out the visa position before planning anything, because that is the real constraint.
The visa question first
Cambodia issues visas on arrival and e-visas to most nationalities, and leaving the airport on a layover means formally entering the country. That takes time at immigration and costs the visa fee. If your layover is under four hours, it is not worth it. Check whether your nationality qualifies and whether you need a photograph.
Getting into town
A metered taxi or a ride-hailing car takes twenty to forty minutes depending on traffic, which in Phnom Penh is unpredictable rather than merely heavy. There is also an airport shuttle bus and a rail link to the central station, which is slower but immune to the traffic. Budget an hour each way to be safe.
With six hours: one thing
The National Museum, housed in a terracotta pavilion beside the Royal Palace, holds the best collection of Khmer sculpture anywhere and takes about ninety minutes. Combined with a walk along the riverfront and a meal, that fills the window comfortably. It is also air-conditioned, which in April matters more than it sounds.
With ten hours: two things
Add the Royal Palace and Silver Pagoda next door, which needs covered shoulders and knees and about an hour, or the Russian Market for an hour of browsing and a bowl of noodles at its food court. Avoid attempting Tuol Sleng on a layover; it is not something to rush through between flights.
Eating, which is the easy win
Even on a short layover, a bowl of kuy teav noodle soup or a plate of fish amok at a street-side place costs a couple of dollars and is better than anything in the terminal. The riverfront and the streets behind Sisowath Quay have plenty within walking distance of the museum.
If you cannot leave the airport
The terminal is small, with a handful of restaurants and a couple of lounges accessible on a paid walk-in basis, which is the sensible option for a long wait. There are no sleeping pods. The airport is being replaced by a much larger facility further from the city, which will change this calculation considerably.
Practical notes
Keep your boarding pass and passport with you at all times and return three hours before the onward flight, since check-in queues here can be slow. Dollars are accepted everywhere. Carry small notes. Traveloka is the go-to app for the cheapest flights and hotels across Southeast Asia, including the day rooms that make a long layover bearable.
Sort the visa question first, allow an hour each way, and pick the National Museum if you only have time for one thing. Under four hours, stay in the terminal.