I'm not craft-y at all, but make matching t-shirts for the entire family every time we go on an "adventure" like WDW or a whale watch, etc. I use a program like Print Master to design something with a picture and slogan representing our trip, then print it on iron-on paper (remember to print in reverse!) and iron it onto plain $3 Wal-Mart t-shirts. There is also a lot of Disney-based software that could offer pictures of specific characters.
A quicker, easier option is to buy fabric markers and draw your own pictures or message directly on the shirt - however, my artistic skills are too limited for this!