Depends on the audience. Is it kids, adults, mixed, etc.? But, I think when anyone sees a ukulele they automatically want to hear "Somewhere Over The Rainbow/Wonderful World." "Pineapple Princess" is always fun and everyone loves "Hey Soul Sister." But, look at your audience and do what you want to do. If you have fun then your audience will have fun.
I'm guessing I need something jazzy, some nice quick fingerpicking stuff, as an intro. Somewhere along the way, something with an unusual strum as well.