There was lots of cutting up bits of random craft supplies we had lurking around. Construction paper, tissue paper, bits of felt, a few napkins.
While that was going on I got out one of our sheets of pilfered Costco cardboard from the garage and used a marker and some string like a giant compass to draw out the rainbow.
Once I had cut it out, I labelled each band with the colour it should be, both with the word and in that colour. The kids got their own little pot of glue and paintbrush each and they spent quite a while sticking on all the coloured bits and bobs.
After lunch, when it was dry, the kids played around with it like a tunnel, driving cars under it and crawling through themselves.
Later that evening my two helped me set up a blue fleece and a green towel to make a back drop, then we stuck on white clouds and a happy sun. The finishing touch was the mountain of soft toy stuffing that was liberally thrown around. My kids love to make a mess. It just makes them so happy. Luckily these days they are finally into actually helping to clean it all up afterwards as well. Youngest Filth Wizard claimed to be a "cloud princess".