Gullfoss is absolutely gorgeous. Gullfoss is a stop on the Golden Circle Tour run by Reykjavik Excursions and is known as the most beautiful waterfall in Iceland. Gullfoss is located about an hour's drive east of Reykjavik and the water is very cold.
The fall never freezes over because of the fast running water, but the rocks around the fall are very slippery. There is a path leading down to the waterfall and it's fairly muddy, but well worth the effort walking the path. You can walk right up to the water and touch it, but be careful as there really are no safeguards to prevent you from falling into the water.
Nevertheless, I had to touch the water and I can tell you, it's very cold. In the winter and early spring the mist from the falls actually turns into snow because of the low temperature.