Waterfalls, black sand and geothermal water, all within a day's drive of the airport.
This is a shape, not a schedule. Each day has two anchors and a lot of slack, because 3-day itineraries that book four things a day fall apart the first time lunch runs long. Move things around freely — the neighbourhoods are grouped so that walking between stops actually works.
Iceland is the trip where splitting the car cost changes everything: four people in one rental turns a very expensive destination into a reasonable one. Sort the driver rotation and the fuel-splitting rules before you land.