Keflavík is the town most visitors to Iceland see first — Keflavík International Airport, which handles most of the country's international flights, is on the Reykjanes Peninsula just outside town. Keflavík itself is a fishing and service town of about 15,000 people, unpretentious and functional.
The peninsula's main draw for short-stay visitors is the Blue Lagoon geothermal spa, about 20 km from the airport. The milky-blue silica-rich water is warm year-round and has become one of Iceland's signature tourist experiences — book well in advance as it gets crowded. The spa is conveniently located between the airport and Reykjavik, making it easy to stop on the way in or out.
Keflavík is connected to Reykjavik by the Flybus and regular buses (about 45 minutes). Most visitors push on to the capital, but those who pause will find more than they expect. The Icelandic Museum of Rock 'n' Roll is a genuine surprise — a serious, well-produced museum about Iceland's outsized pop music legacy, from the 1960s US military base days through Bjork and Sigur Rós, and admission is free. The Viking World Museum houses a full-scale replica Viking longship that actually crossed the Atlantic in 2000. Out at the tip of the peninsula, the Garðskagi Lighthouses offer dramatic coastal views and excellent birdwatching just 15 minutes from the airport.