You might also be doing something messy for philanthropy day... we didn't have a philanthropy round when I rushed, but from what I've seen since, you might be using paint, glue, magic markers, etc. to make something, so you want to wear something that you don't mind so much if it's damaged.
I'd still suggest something cute / nice looking, I'm not suggesting jeans and a ratty old t-shirt
Congrats on your invites