Seeing cagers stuck in traffic, "Ha ha! Suckers."
I usually have a song floating around in my head that takes turns with other random thoughts. 'Bicycle Race' by Queen gets regular rotation, it's just a nice light-hearted tune. Sometimes I end up singing out loud. Then I think how silly that must look (not to mention how bad it must sound) and I tell myself to shut up. Then I realise that I just told myself to shut up out loud, and think how silly that must look and I start laughing, call myself a dork or something and eventually start singing again. This ends up repeating itself in some form or another for the duration of the ride. So, I find myself riding my bike, singing, laughing, and talking to myself... It's perfect, no place I'd rather be.