If you're only there 3 days Moab would be a stretch...it's more like a 6 hour drive, not 4. It'd take up your whole 3 days getting there, doing something, and getting back. Southern Utah is a destination in itself apart from Salt Lake, and vice versa, and I personally don't think you can do both in only 3 days.
Hiking...easy access would be City Creek Canyon, Ensign Peak, and Bonneville lakesh
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