There is not much to see in my opinion. You should be done in four or five days tops.
You should take a walk around the 'riverside' area along the Mekong river. Check out the cool restaurants around there. Also, I would suggest walking along the Norodom Boulevard, which is one of the most developed areas in the country. The Independence Monument is on the way too. At the north end of the highway, there's Wat Phnom, an interesting temple with a small garden area. about 8 years ago
There are also several interesting markets to check out -- the usual tourist destinations of the Central Market and Russian Market are fun, but for a more unusual (and hectic) experience try the O'russey market. There are some other good bars and restaurants at St. 272 (off 51). The Master Suki Soup on top of the Sorya mall has a great view and is fun for a meal.
You could also try the Nagaworld casino complex or one of the fancy hotel buffets. Don't miss some great cheap massages.
I also highly recommend a day trip out to Phnom Tamao - the wildlife sanctuary. You will see amazing animals very close, including tigers and loads of bears, and it's a great cause. Almost all of the animals have been rescued from traffickers. (And they do a lot with little money.) about 8 years ago
You should consider watching Cambodian Boxing (similar to Thai Boxing) at one of the television studios around the city. The fights take place on Friday evenings and weekends. Great way to experience the Khmer Culture.
I have more info about Cambodian Boxing on my website: http://tinyurl.com/43z5nex
I tweet about Cambodia, feel free to follow me on twitter @tropasiatravel
Have a great time in Cambodia. about 8 years ago
If you're interested in seeing/supporting local sites and businesses, I recommend checking out this guidebook: http://friendshipwithcambodia.org/programs-education.php. It takes you a little bit off the beaten track. Plus, it highlights local businesses that are socially responsible (i.e. do not support prostitution, use resources ethically, support street children in a responsible manner, etc).
(Disclaimer: The friend who gave me this guidebook before I went to Cambodia volunteers with the organization that publishes it.) almost 8 years ago