I'll pitch in because no one else said it-basic paper and a glue stick.
Office stores sell them in bulk for us teachers (thank God!). Anyway, I print my labels on an inkjet, so they run a bit when wet. You can write in sharpie or print on a laser printer (to avoid the watercolor effect from condensation), cut them out to size and use the glue stick to paste 'em on. Comes right off in the wash. And then you can recycle/mulch the paper if so inclined.