Here is a short list of stuff I think about when I'm alone...
If people really do see the world differently (litteraly that is)
What it's like to be color-blind
Why I don't care at all about music
What the values that we all learn by the age of ten were in my case, they are hard to see
What I really want to do
What other people are doing, at the exact same moment
What my cat is saying when he walks around crying
Why animals have such diferent personalities even when they were born and lived with one another their whole lives
What my twin would be like if I had one
How the world is actually going to end
Why is somewhat dislike everyone I meet
Why is water tasteless to us, and what would it taste like if we were from somewhere else
What it would be like to walk around on the moon
What my voice actually sounds like, not just on a recording but in real life
Different machines I could use to make my life easier
If I had a clone and could make it do anything I wanted so I wouldn't have to: would I want it to do stuff for me? What would I miss out on? How would I change as a person?
How magic tricks were actually invented, and why people get so crazy about them
A lot of other random stuff.....