I bought an Eric Carle coloring book that came with stickers. I had my older kids color and hung them up. Then used the stickers on little construction paper cards I used to label the food. Just check Pinterest for easy stuff. Tape some red and green balloons together or use colored paper plates and call it a day.
I just made my own. Here's a couple pics. The month by month photos were pretty easy (if you have monthly photos to print out, anyway) and very popular.
I am not great at decorating cakes, so I printed these, used a circle cutter, glued them to toothpick/skewers and used as cupcake toppers:
We also put up red and green balloons and had red and green paper plates/napkins.
I've been to 2 hungry caterpillar birthday parties and both moms have commented that they thought it would be so much easier to find stuff. One said she found all sorts of stuff online, but not sold in the U.S. and she wasn't going to pay international shipping for cups & napkins. The other just used some Polk a dot stuff she found at a party store.
Zulilly recently had stuff; not sure if the sale is still there. Or it might come back again soon. Books a Million had some random Eric Carle stuff when we were there a few weeks ago. Not necessarily party stuff, but a bunch of little things.