Reykjavik is a small city that is easy to explore on foot. However, if you prefer to use public transportation, you can purchase a Reykjavik City Card for access to all the city's buses and public pools, as well as discounts on various museums and attractions. The card costs 3200 ISK for 24 hours.
If you have more time, consider taking a day trip to see some of Iceland's incredible natural wonders, such as the Blue Lagoon or the Golden Circle. If you have less time, you may want to skip some of the museums and focus on the outdoor attractions around the city, such as the Tjornin pond or the Perlan observation deck.