I hate to be a wet blanket, but I was slightly offended by the Easter shirt. Offended isn't always bad, though.
Anyway, I thought a good shirt design would be to have a wind up duck on the front and on the back have a saying about the duck being able to find things or whatever.
Edit: I'd also like to have a shirt (despite getting annoyed by its constant use in book 2 and 3) that says "Hell's Bells" on it.