The vastness of space specifically. I absolutely love looking up at the stars, which is something that is unfortunately not very satisfying where I live (near a city and an airport!) due to the light pollution :( Visiting my mum's house has the added benefit of a brilliant night sky where I can see hundreds of stars.
But the best view of the night sky I have ever had was in Australia. I spent about 10 weeks backpacking there some years ago, and one of my favourite memories (and I have a lot of great memories from that time) is the night I spent on a boat around the Whitsunday Islands.
The boat's skipper was a trained Celestial Navigator, and after sunset he offered to give a little talk at the back of the boat. Only a few of us joined him, most of the other passengers were more interested in the bar *laughs*
They really missed out. The sky was so clear, I've never seen anything like it at all. The skipper (whose name sadly escapes me) had a pair of incredibly powerful binoculars and let me use them - on the basis that I hadn't had any alcohol and the binoculars cost thousands and thousands of pounds ;)
As I looked, he talked about how the stars on the horizon seem to twinkle more than the stars higher in the sky, due to the way their gases interact with our atmosphere. He pointed me to the stars he was talking about and... Wow. They flashed red and green, like disco lights.
The skipper pointed out all the constellations that we could see, and told us the legends of how they came to be. We saw Scorpio (my favourite constellation) and he told us the story that one of the Gods had decided that Orion was getting too big for his boots, so he sent a crab to kill him. The crab, Cancer, failed so the Gods sent a scorpion. Scorpio succeeded, but one of the other Gods brought Orion back in the stars. For this reason, Orion and Scorpio are never in the same hemisphere and cannot be seen in the same sky. I love this sort of story, and I will restrain myself from telling all of them here, because I could go on for hours *laughs*
Amongst the star gazing and moon gazing (very bright through powerful binoculars by the way!) one of the other people who had come to listen to the talk said something like, "it's so clear, apart from that one weird cloud" and pointed to a smudge across the sky. The skipper laughed and told her, "no, that's the milky way".
It was completely breathtaking. I honestly don't have the words to describe what it was like, sitting on the back of a boat, on the other side of the world from my home, looking up at this incredible sight that I never expected to see.
This isn't my picture, but it's the closest I can find to what it looked like - although, really, it doesn't come close!