Visiting the Easter Island is going to be such a special trip that any little souvenir will bring back incredible memories.
My advice is - carry your passport around all the time!
Especially if you're a passport stamp freak like me. The two quarries of Easter Island, Rano Raraku and Orongo, are both National Parks and both will stamp your passport if you wish so!
I think that's pretty special and I love looking at my passport over and over again, thanks to the one stamp I got at Rano Raraku (I had no idea about it when visiting Orongo, what a shame!)