Preah Vihear is a good trip, but Thailand and Cambodia still engage in occasional sabre-rattling over ownership (the Thais never really accepted the World Court decision awarding it to Cambo) and access from the Thai side is closed off randomly. Make inquiries before setting out on the long haul to Ubon Ratchathani, the nearest city... and don't miss Wat Lan Khuad ("Million Bottle Temple") while there.
http://wikitravel.org/en/Preah_Vihear