I love Moo for printing - and am thinking about using the stickers this year... not sure how. Last year I used photos my students took on a nature walk to make mini moo cards... on the back I had our class blog info and a place to add a descriptive phrase inspired by the image - we used them for a collaborative poem. Once we put the cards together to read the poem, we flipped it over to see the collage of images.
I don't have a rewards system in my class... but I have let the kids make stickers with a Xyron sticker maker. I punch out shapes and let the kids draw - couple of seconds later... a sticker. Incredibly easy and inexpensive.