Hum riffs from my favorite songs in my head. A lot, I'll space out at school, work, even talking to friends because I got songs running through my head. I'll also talk to myself in my mind, which I hope isn't that weird. Like I'll just ramble about whatever I'm thinking about and I'll sometimes lose myself completely in my own thoughts and arguments.
"I start having discussions with myself by assuming 2 different roles on 1 stance but people look at me weird when I do that, I also walk on the edge of the sidewalk trying to balance myself, and people look at me weird for that too..."
Looks like I'm not the only one. Whew, that makes me feel less strange.
EDIT: Wikipedia. That is God's gift to insomniacs. Any random thought I get, wikipedia.