Here´s what you do
Both local Icelandic airlines, Icelandair and Wow Air, offer transatlantic flights with the catch you make a stop in Keflavik airport.
Depending on the time on the ground you might be able to squeeze in something more memorable than airport wait.
The big question then becomes if you have time to do anything of value with the time you have.
Check out the suggestions below, usually 4 H plus is good.

1. Blue Lagoon

The Blue Lagoon, one of 25 wonders in the world and on the list for the most popular visiting spot in Iceland.
you will need to have a booked reservation but it will be worth it relaxing in the water between flights.

http://www.bluelagoon.com/

2. Gunnuhver geothermal area

30 minute drive from the airport.
Take a rent a car and go for a short drive around the peninsula
Gunnuhver geothermal area is near the airport and you won't be
disappointed with the nature view there. item 3 Reykjanesviti is rely close by

3. Reykjanesviti

30 minute drive from the airport
Iceland's oldest lighthouse ,The lighthouse is located near an area of thermal activity Gunnuhver,
the surrounding cliffs are full of seabirds and from the shore you can see Eldey island, home to one of the largest gannet colonies in the world.

4.The bridge between continents

21 minute drive from the airport.
The lava-scarred Reykjanes peninsula lies on one of the world's major plate boundaries, the Mid Atlantic Ridge.
According to the continental drift theory the Eurasian and North American tectonic plates are continuously drifting apart with great forces under the gaping rifts.

5.Viking world

Viking World is located at the seaside in the town of Reykjanesbær with a spectacular view of the Atlantic Ocean.
Viking World is a five exhibition museum along with a settlement zoo, a playground and an outdoor classroom.
The biggest attraction of Viking World is the magnificent Viking ship
http://www.vikingaheimar.is/en