I did a one day layover in Iceland about 4 years ago on a business trip. 10/10 would do again. I went in May and didn't see the lights, but had a blast.

A couple of key points.

1. Flights are actually pretty cheap. It was $250 one way (4 years ago).
2. Renting a car and driving there is pretty easy. I rented a cheap car and drove around all day without issue. No traffic, drive on the same side as normal.
3. Work with you cell phone provider ahead of time for international coverage. Its like $5 or $10 a day if you coordinate ahead of time.
4. Driving around and looking at moss, mountains, waterfalls, etc. is great fun.
5. Blue lagoon requires reservations, but its not the only game in town.
6. Capital city is like 200,000 people with all major sites within walking distance of their cathedral.
7. You can get some different food only legal there. I had a nice whale steak served over potatoes. 10/10 would recommend.
8. If you like any of Tom Clancy's stuff, Red Storm Rising was his first non Jack Ryan alternate timeline book. WW3 with Russia, but it prominatly features a dude trekking on foot across iceland roughly from the airport to the main city. Was kind of a neat tie it.
9. Lots of cool hiking and biking if thats your thing.
10. Tourist traps are traps and are expensive.

I'd do it again, but would recommend going when the weather is a little nicer. I've had family fly out and do a cruise that stopped in iceland that prominatly featured the Northern Lights if thats you thing.

Good luck!

Northern Lights ? That's what we have attract mode in pinballs for!

Kidding I live in Canada they are really are beautiful to see , have a great visit.