Apart from simply getting on the next bus to Reykjavík, there are several options to get out from Hafnarfjörður. You could take the road south towards the lake Kleifarvatn and Krýsuvík, a geotehermal area on the south coast of Reykjanes which in fact belongs to the municipality of Hafnarfjörður. You could also drive west on Reykjanesbraut towards Keflavík about 20 minutes away and the other towns on the Reykjanes peninsula, or even go on a trip to the Blue Lagoon.