Rano Kau is next to Poike and Terevaka one of the 3 volcanoes, which was responsible for the creation of Easter Island. However, the volcanoes have all extinct, so you do not have to worry about volcanic eruptions.
Rano Kau has, with about 1.6 km in diameter, the largest crater on the island. It is located near Hanga Roa and is accessible by vehicle. However, I would recommend to hike the volcano via the “Te Ara O Te Ao” trail. If you reach Rano Kau via this path, you directly have a nice viewpoint to look into the crater. As well there is a stone with a very visible birdman petroglyph.
You can combine your visit to the volcano with a visit to Orongo. The place of worship, known for the birdman cult, is located at the edge of the volcano.
The crater of Rano Kau is a bit broken on the sea side, so it is not possible to walk around the volcano completely. However, you can go around 75-80%, starting with Orongo on one side, to a falling edge on the other side. I would also recommend doing this, because you have great views over the Easter Island on the way. The endpoint also offers a nice photo opportunity on the small offshore island.
The crater itself is partially filled with fresh water and was an important reservoir for the island. This freshwater lake has a unique flora due to its location inside the volcano. It is strictly forbidden to walk down into the crater, but the steeply sloping walls here also form their own protection against incorrigible people. The crater is one of the photo opportunities that look great on photos, but still cannot transfer the reality. If you stand in front of this 1.6km wide and 100m deep crater, Rano Kau is even more impressive than in any photo.