This question is probably a bit too open ended. What are you interested in? Can you afford to hire a guide/driver or are you planning to just wing it on your own with tuk tuks? Do you want to spend time seeing a million temples? It's easy to get templed out unless you are really fascinated by them. There are tons of them besides Angkor Wat. Some of the more outlying sights are a good bit less crowded than the main attractions.

My last couple of visits through Phnom Penh I found the traffic to be really unbearable. It's grown tremendously in the last few years.