Wake up early and go visit the Royal Palace. It is well worth $10 entrance fee and reminded us of the Royal Palace in Bangkok.
After that, go visit Killing Fields and S-21 Prison. Those are two very emotional places to visit and, to be honest, we had a hard time. Well, Linda(me) is also a very emotional person. Definitely take the audioguide which costs only $3 extra the $3 entrance fee. The audioguide is pure facts and will leave you trembling after the realization of the terrible recent Khmer history. Tuk-tuk can take you to both places on the second part of the day for around $15 total and you will be back by 5 pm.
Steepshot