Angkor Wat is an incredible place. You have the big main temple, which is great to look at. However, I much prefer the smaller satellite ones - especially the one with the overgrown trees. That is amazing.
The closet town is Siem Reap, however there isn't really anything else there to warrant more than your visit to Angkor Watt. Depending on how much of a tourist buff you are, you could get away with one night in Siem Reap, and spend the whole day at AW.
There is nothing in Phnom Penh, apart from the crazy traffic (I would go there just to try to cross the road once). Apart from one temple and a poor museum, then there is nothing else worth staying more than a night for. You can drive (get driven) to Siem Reap - it'll take about 6 hours and it is an experience. Cambodian folk are really friendly.
I was seriously disappointed in Ho Chi Minh City - don't like the city, the people or the attitude. I haven't been anywhere else in Vietnam, but I doubt I'll go back to find out if it is better.