I think I have this figured out. We are going to rent a compact car at Sixt; all rental cars seem expensive but it is due to "one way" rental as dropping off at airport. Sightseeing plans are:

Stop at two of south Iceland's most impressive waterfalls, Skógafoss and Seljalandsfoss, the former being one of Iceland's most powerful waterfalls. Seljalandsfoss is a picturesque cascade, famed for its walking path which allows you to hike behind it in summer time.

Then we will go to Reynisfjara, a black sand beach. This beach is famed for its vibrant birdlife and eery natural surroundings.

During the drive we will see both the Sólheimajökull glacier and the Eyjafjallajökull glacier volcano whose 2010 eruption brought European air traffic to a standstill for days due to the enormous amounts of ash it spewed into the air.

Next to Blue Lagoon before dropping off the car at Keflavik spending the night at a hotel at the airport.

If anyone is on similar schedule and wants to see same sights we can probably combine in a larger car (luggage might be a problem). Marcie, sorry, this won't work for you as it will take a full day.

Glad this trip is finally getting close.

Marc