Vayyinafash can’t mean that God literally rested: He is not a physical being who is tired from bodily exertion. The rabbis link vayyinafash with nefesh, a soul, and say that on the seventh day God was on a spiritual high (though even that is hard to grasp unless we recognise it as poetical metaphor).
Resh Lakish said that God attached a n’shamah yeterah, an “extra soul” to Shabbat, giving all who observe the day an inrush of spiritual feeling that is unlike any other emotion.
When the Christian critics of the Jewish Sabbath accused it of being dark, burdensome and restrictive, Solomon Schechter wrote that this picture is totally foreign to the Jews he knew who observed the Sabbath in their own lives. The R and R of Shabbat suffuses your soul to such an extent that your life is not just inspired but inspirited.