The day of arrival or departure is often spent waiting either for check-in at your accommodation (if you fly-in in the morning) or waiting for the check-in at the airport (if you fly-out in the afternoon/evening). Personally I do not particularly like these days and are usually less fun and less (good) memorable than the rest of the trip.
That is why I am posting an alternative to the boring-waiting-days that we are all used to on travels. The directions will be written like you are arriving to Iceland but this might as well be your day of departure and make it a reverse direction.
First, do not take the main route to/from Keflavik Airport but make your way towards Hafnir via Flugvallavegur and Hafnarvegur  and later Nesvegur  which leads you to the Bridge Between Continents.
After your walk between the American and the Eurasian Tectonic Plates 🙂 you make your way to Reykjanesviti, and the cliffs close to it, and in the same area Gunnuhver Geothermal Area is a must visit.
Further along Nesvegur  route you visit Brimketill
Next (super exciting) stop is the Blue Lagoon but before that you should (if you are hungry) stop at Bryggjan Restaurant/Café for their soup and would like to try something more authentic than the food offered at the Blue Lagoon.
So this is where it comes a little tricky. You need to reserve you spot in the Blue Lagoon via their website well in advance. If we assume that it takes 1.5 hour to get out of the airport and perhaps additional 2.5 hours, with stops at the sights above, 4 hours after touch-down should be fine. If there is a little delay of your flight you can always cut this day a little shorter and make less stops to get to the Blue Lagoon in time.
You should account for around 2-2.5 hours in the Blue Lagoon but if you choose some treatments obviously it will take more time.
After your visit to the Lagoon you drive back to Grindavik and further along the south part of the Reykjanes Peninsula via route 427 towards Krýsuvík. During summer months the Krýsuvík Cliffs are a must visit but during other seasons you should be very careful and you should not be surprised if the road is block or inaccessible once the snow starts to fall.
If the road to the Krýsuvík Cliffs are not accessible make your way to Grænavatn, Kleifarvatn and Seltún Geothermal Area. All interesting and easily reached places.
At this point you should be used up most of the time before check-in so make your way to you destination.
The Reykjanes Peninsula is often "forgotten" tourist attraction despite being so close to both Keflavik Airport and Reykjavik. If you are not driving yourself you can either contact us through the questionnaire form og via email: firstname.lastname@example.org and we will help you find an operated tour for this day 🙂