There are two other Avis office locations in Reykjavik, in case you're flying on to other cities in Iceland and won't be returning. You can drop off your vehicle at the Downtown branch, which is located at Holtavegur 10 in the central Holtagardar district. Even more conveniently, you can also return it to Reykjavik Domestic Airport, which handles all flights from Reykjavik to other cities in the country. Both of these branches offer 24-hour drop off, so you can return your vehicle whenever suits your schedule.