Traveling to and from Keflavík International Airport is rarely a hassle. There may not be any trains in Iceland, but the roads are not congested and, the airport shuttle is almost always on time.
There are several coach services to choose from, the best-known option being FlyBus. The reasonably priced service runs in and out of the BSÍ bus terminal in Reykjavík, but for a little extra they will drop you off at your hotel or hostel, or the domestic airport. The bus also makes a scheduled stop in Hafnarfjörður on the way.