You can walk behind Seljalandsfoss.

Wear a coat. Just a hint.

A million people come here every year. Bit of a muddy path. Very pretty, though. 
Tour busses everywhere. Wait your turn.

Or you can sneak off to the Kistá, on Snæfellsnes, which has trolls. Real trolls. You can see a big one lurking in front of the fall below. Sorry, not on the must-see-waterfalls of Iceland lists. But it’s just off the road. You can sniff it out on the edge of the Berserker Lava Field. 

Bit of a brown place in November, but it greens up real nice in the summer. Oh, and there’s a second troll leaning over the cataract, so a bonus!

Nice. If you were four metres tall you could reach high tup from your waterfall lair and scratch her under the chin, even! Oh, yeah, one more thing. Bit of a muddy path. You can approach the falls from both sides, but only on the north side can you get underneath. Sorry, no crowds.