First, whatever you decide, you must do it soon. April 14 is not that far off, and people need time to order and receive their T-Shirts.
Here's a thought that works well for a local breakfast spot. They give out free bumper stickers. If you bring them a photo of your car sporting the sticker in an unusual place (for example, next to the entrance sign to the Grand Canyon) you get a free breakfast.
You could ask people to email photos of themselves wearing the Ts in various situations. Prizes could be awarded by first-come, first serve, or by your assessment of how cool the shot is, or by CB vote.
This gives lots of people a shot, gets the T shirt exposure in lots of places, and encourages creativity.