Snæfellsnes peninsula
Full day · ~2 h each way
"Iceland in miniature": one easy paved loop collects Kirkjufell — the arrowhead mountain with its own waterfall — the little black church at Búðir standing alone in a lava field, the basalt arches and bird cliffs at Arnarstapi, and golden-sand Ytri-Tunga where seals haul out, all circling the Snæfellsjökull ice cap that Jules Verne picked as the door to the centre of the Earth. Quieter roads and thinner crowds than anywhere on the main plan.
Swap for → a whole day, not a slot — trade it against the Golden Circle (Day 2) or the South Coast (Day 3).
Bump note: the gentlest big day out — flat viewpoints and short strolls at every stop — but the longest gaps between bathrooms of the three loops; top up at the Borgarnes N1 both ways and keep the snack bag loaded.