Your first day starts in Reykjavík. You are going to follow the Ring Road (road number 1) in eastern direction (direction Vík).
Your first stop is The Lava Tunnel. This experience is going to lead into the mountain and in one of the tunnels which has been formed during the Leitahraun eruption.
After that you are going to allay your hunger in one of Icelands food halls in a small town called Hveragerði. In the so called Gróðurhús food hall you can choose between many different types of food.
Your next adventure of the day leads to some natural hot springs in the Reykjadalur valley (smoky valley) which got its name after the numerous natural hot springs located in the valley. You keep your car in the parking lot and follow the hiking path leading to a hot stream which has the perfect temperature to take a dip in.
The last sight of the day is Kerið crater. A crater close to Selfoss which is known for its red colored ground and the lake in the middle of it.
To end the day I recommend to dine in Kaffi Kruz. This restaurant offers good food in the middle price category. Other nice places to eat in Selfoss are the food hall called Mjólkurbúið and the restaurant Crisp.
I recommend to stay in Selfoss for the night.
