This is where you can walk between two tectonic plates, the only place on the planet where you can do so on land. There is also a lot of history to learn from this place, the museum inside the visitors center is work the small price if you want to learn about Icelandic history. There is a nice walk through the park, very easy and paved to see some waterfalls and other interesting sights.
If you are used to National Parks in the US, this is nothing like that. But it is still beautiful with an easy walking path to a waterfall between the tectonic plates.
There is a cafe inside if you need a snack and a small fee for parking.