Take a morning or evening stroll by the riverside and see the locals go about their daily lives.
There are endless shops to hop in and out. Countless cafés and bars to choose from. Serving all sorts of cuisine from all around the world. My favourite so far is FCC {Foreign Corespondents Club} on 363 Sisowath Quay. Impossible to miss it. Good food and excellent view of Tonle Sap river.
Another alternative is to hire a boat and enjoy a relaxed sunset cruise on Tonle sap and Mekong river with a photogenic view of the Royal Palace and PhnomPenh skyline. Boats are easily available and rates are affordable. Plus you have an option of bringing your own drinks and food. 
