Here my suggestions for trip in Riem Reap Cambodia.
Three and halft days still so sort time around Angkor Wat complex but anyways you can spent two full days that's fine, In two days visit you sould and don't missed to visit Banteay Srey temple(The pink sandstone) about 37km from Angkor Wat! another one more day around town visiting and floating Village. In Siem reap have there are 4 floating Village such as ChongKneas, Kampong Pluk, Kampong Kleang ...