Reykjavík's scale is deceptive. You can walk the entire downtown in forty minutes, and yet it contains more good restaurants per capita than most cities ten times its size. Plan on three days: one to acclimatise, one for museums and swimming pools, and one to start the journey east.
A pool, not a spa
Skip the Blue Lagoon — it's thirty miles out of town, expensive, and crowded. Go to Sundhöllin or Laugardalslaug instead. These are neighbourhood swimming pools where Icelanders actually swim, with geothermal hot pots at varying temperatures along one edge. The etiquette is strict and the water is perfect.
What to eat
Icelandic cuisine is having a moment, and you should lean into it. The cod is pulled from the harbour that morning; the lamb has been grazing on wild herbs all summer; the rye bread is baked with geothermal steam. Sample all three, then walk it off along the harbour at dusk while the light goes silver.
When to leave town
Don't stay in Reykjavík for more than three days. The real reason to come to Iceland is the Ring Road — a 1,332-km loop that circumnavigates the country and delivers a new landscape roughly every ninety minutes. Rent a car, book guesthouses in advance, and budget a week.