Iceland's best-known tourist site sits in a black-lava field forty minutes from the city on the way to Keflavík airport. The Blue Lagoon's pale blue water — the colour comes from silica suspended in it — is a by-product of the neighbouring Svartsengi geothermal power station; the temperature stays around 38°C year-round.
Book ahead, weeks ahead in summer. Tickets include a silica mask and a drink at the swim-up bar; upgrades add a robe and a meal. It is touristy and expensive but, particularly under snow on a winter night with steam rising off the water, it does live up to the postcards. The Sky Lagoon and the city pools are cheaper alternatives.