On the tip of Seltjarnarnes Peninsula, is a place called Grótta and there you'll find a popular lighthouse. Grótta was declared a nature reserve in 1974 due to its abundant birdlife. The lighthouse was built in 1897 and later, in 1947 there was another one built close by. In recent years it has become one of the most visited sites in Reykjavik area, being an amazing place for walking, biking or bird watching. Be aware of the fact that the lighthouse is accessible only during low tide, although it is still visible from distance if you go there during high tide.
Grótta is also a popular place for people to visit when they want to see the Northern lights. If the weather is good with no clouds, and the solar activity is high, Grótta is a good spot to view the lights.