I bet you’re very excited to see what Phnom Penh has to offer. As someone who grew up there, here are the 3 spots I would suggest for your first day.
Royal Palace
The Royal Palace locates in the best part for the city, right next to Mekong River. I recommend it as the first day trip is because this place in mostly quiet and less hectic.
National Museum
After the Palace, you can easy take a 15 minutes’ walk the museum and check out all the artifacts and in one place.
Central Market (Phsar Thmey)
This place can be your first or last day of the trip. If you can squeeze this into your first, it’s not a bad idea to learn about the local people because this place is pack with both local and tourist. And if you decide to save this for the last day, it’s a good spot to get all the souvenirs you need.