Cambodia forum

For that you only have one choice: Sihanoukville. Beer is cheap, accommodation is cheap, plenty of backpackers and always somewhere to go in the evening/night. Beaches are ok in town but the best ones are out of town.
You have a great time there.

Distant second is Kampot with a laid back bar scene in the evenings. It doesn't have a real nightlife scene or beach but plenty of packpackers.

I agree that Sihnaoukville is definitely much more lively than Kampot ....BUT if you want to experience more of the real Cambodia, and less of the party night life/tourist traps, I would highly recommend checking Kampot out. I went there for what should have been just night or two and couldn't get myself to leave (stayed 6 days)! It is very laid back and the people are awesome. Not everyone is trying to sell you stuff and you can actually get to know Khmer people. Also, an idea (if time permits) is to visit one of the islands off the coast! They are supposed to be amazing and very rustic (i.e. only electricity for a few hours a day). You can reach these from Kampot, Sihnaoukville and Kep (a city 30ish min from Kampot). I regret not allowing myself the time to check one out for a night! I know there are a couple of options, but I can't really recommend one. Usually the best way to do it is to ask around for suggestions and see what other travelers have to say about it!

Hope this helps! Enjoy your trip, it is going to be awesome!!