If you are visiting the temples in Siem Reap I would recommend doing a day trip as it is plenty of time unless you have a serious interest in the temples and tolerance for heat.
Make sure you go early to catch the sunrise. Usually if you book a decent Tuk Tuk driver they will collect you from your hotel/hostel around 4:30/5 am. Once you get there he will bring everywhere you need or want to go…its one of the better day trips in Cambodia. Take plenty of water, sun cream and DEET with you. some of the best advice for the day trip can be found here