Drape rainbow ribbon (from $15.50, artisticribbon.com
) across a plain white tablecloth (you could also let the guest of honor decorate butcher paper with crayons or markers). Use plates and cups in primary colors. Send home nail polish and handmade jewelry and accessories (see Activities, below).
Have guests wear their most colorful outfits. Or ask them to come dressed in an assigned color of the rainbow, then at the party, line them up against a wall, rainbow-style, for a photo. Give rainbow manicures (each nail a different shade), and make friendship bracelets with multi-colored beads or headbands (look for wide rainbow elastic at a fabric store, and attach feathers and beads with fabric glue).