If you are doing the Golden Circle and visit the Gullfoss waterfall you might consider doing a small detour. If you are heading to (or from) the Gullfoss waterfall along road 35 for about 15-20 minutes you will be able to stop along the side of the road. Close to the road you will find another waterfall called Faxi.
Faxi isn’t anywhere near the size of the Gullfoss – but it compensates for the small size by the lack of tourist. Here you can enjoy the waterfall privately unlike the situation at the more famous cousin close by.
The falls isn’t a big enough attraction to make it all the way out here by itself. But it is very close to both the geyser thermal area and the Gullfoss falls so you could make this your bonus stop on a trip out here.