The park was the meeting place of Iceland's first parliament in 930 AD, where various chieftains met to discuss law and matters raises forth by citizens.
It is also the meeting place of the Norh American and Eurasian tectonic plates, which are in the (slow) process of separating from one another. This causes deep fissures and rifts. We actually saw a few scientists out measuring the width and depth of some of the smaller cracks.
The spa is something between the Blue Lagoon and the public pools in Reykjavik - nicer, yet pricier than the public pools but cheaper and less touristy than the Blue Lagoon. The spa features multiple pools of varying temperatures, a sauna, and three different steam rooms, heated entirely by steam from the ground. The entire property is situated on a thermal lake that can sometimes be as warm as boiling (but was chilly while we were there). Like the city pools, none of the water is chlorinated, and one of the pools is a "natural" pool with with rocks and algae and mineral water.
I'm sure it was fabulous in its heyday, but Geysir stopped erupting after being rattled in an earthquake. Fortunately, a nearby geyser, Strokkur, is still very active, squirting giant, boiling bursts of water every 10 minutes.
The two of us are mega scardy cats when it comes to heights and one of us (hint: not me) had a minor meltdown at the thought of walking around the perimeter. After a lot of coaxing and a few tears we both made it. It turns out that the trail around the crater's rim is wider and safer than it initially looks but it is a little disturbing that there are no rails or ropes or anything to keep you from tumbling in (this seems to be common theme in Iceland).
You can also walk down to the actual water but it's a steep walk, and the view is significantly better from above.
I am SO proud of my mom for making it around the rim! It was no small feat but she conquered her fear, which is huge!