My favorite random celebrity sighting was a few years ago when my sister and I were at a reggae concert at the House of Blues and Damian & Julian Marley (yes, Bob Marley's sons) were in the audience.
Turns out they were performing at an outdoor concert in Chicago the next day & were booked at the House of Blues hotel...they found out there was a reggae show that night and decided to watch from the audience. They sent somebody in their crew over to ask my sister and I if we knew a good place to hang out after the show so we ended up driving half of them (the rest followed in another car) to a reggae club in Wrigleyville afterwards. Damian was very nice, laid back & humble.