Absolutely do not abscond. Be reasonably honest.
You’re a part of a network of people, who all know each other, and the higher up you go, the more you’ll realize how important this moment is.
You can say, “After working here for x days, I’ve realized that I’m not the best person for the job. This company (and you) deserve someone who is 100% invested and I’m not. To limit the fall-out of an early departure, it’s best I leave before you give me real responsibilities. Thank you so much for trusting me and giving me this opportunity. I wish this company and you nothing but the best. Thank you.”