You simply cannot visit the archaeological sites on your own. You could drive around the island yourself, but you cannot enter any of the sites unless you're escorted by a licensed local person. The best way to see the sites is to join a quality group tour. See the background page for an explanation. I did an all-day trip with Hahave Rapa Nui and was very pleased with the experience. Try to get on a tour led by Emilia Tepano, she is an excellent guide.