I made a bunch once using the little 1" balsa wood heart cutouts that you can buy in a big package at Michaels'. I painted them white (lay them all out on sheets of newsprint paper and spray them all at once), drew a 3 color rainbow with paint pens and then put first names on with a very fine black paint pen. Glue a pin back on and there you go!

I meant to add that you could do the three color rainbow as a squiggly line diagonally across the heart to make the "Rainbow Road".

Thanks, Jaybird!
We made similar ones several years ago, but painted the stripes with brushes, which I found difficult. I think paint pens would be much easier.

Another option would be to string rainbow colors of pony or other beads and attach them so they hang down behind a wood cutout. You could use various shapes for the wood cutout depending upon what you wanted. You could even glue cotton balls to a cutout for clouds.

I've seen people make cute pins out of shrinky dinks. You could make some and then put glitter glue on them afterwards to make them sparkly.