There are a variety of daytrips to do outside of Phnom Penh’s city centre, such as visits to a local weaving village, Phnom Chisor, and Tonle Bati Lake. Visitors can go from Phnom Penh to a Mekong River fishing village on a small touring boat and visit to a silk weaving village and sunset on the Mekong River. Expeditions like this can be arranged by asking staff at your hotel, through any travel agency in town, or simply by hiring a tuk-tuk guy or moto guy to take you out there. You will have to negotiate the fee witht the driver if you do it on your own.  

One can also visit the Phnom Tamao Wildlife Rescue Centre, 44 km from Phnom Penh. This 6,000 acres of forested area is government owned and operated in partnership with Wildlife Alliance who are working on the front line to protect, rehabilitate and release wildlife rescued from the wildlife trade. The Centre offers behind-the-scenes tours which gives you exclusive access into private areas of the Centre to interact with some of Cambodia's most endangered wildlife, including elephants, monkeys, gibbons, tigers, and baby animals at the nursery. Bookings can be made at phnomtamaotours.com, and all tour profits go directly to the animals' rescue and care. Betelnut Jeep Tours, which operate out of the Lazy Gecko Cafe, offer open-air jeep trips to the sanctuary. Both tours include pickup in Phnom Penh, lunch, and knowledgeable guides. Standard admission, with no tour, is U.S. $5. Government employed guides hired at the Centre's entrance usually have limited knowledge of the animals' rescue stories and a low command of the English language, so arranging a recommended tour in advance is highly advised. 

Another popular visit is to Oudong, which is 41 km north of Phnom Penh. Oudong, the former capital city of Cambodia, is a good way to learn about the history of the area and see remains. You can take a bus there, or arrange a taxi to take you there (upwards of $30 for a round-trip).

To get a glimpse of Cambodia's brutal history, visit Choeung Ek Memorial (The Killing Fields) 15 km southwest of the city. This site can be easily reached  via moto or taxi, depending on how much money visitors wish to pay. The Tuol Sleng "Genocide Museum" is open every day from 8am to 5pm. A $2 entry fee is charged for the Museum and a tour with guide costs about USD$2-3. You can also go for the audio tour. The Killing Fields shows the place where the Pol Pot regime brutally massacred thousands of Cambodians who were considered counter-regime and left their remains in large dug-outs in the ground. The extremity of these massacres was not fully known until after the regime was thrown. While grass-covered pits remain where the bodies were once dumped (the mass graves containing the remains of close to 9000 bodies were exhumed), there is a memorial tower in the field containing a large collection of bones and skulls which was erected in 1988.