So fun!!!!!
Here are my ideas...
I like the collage idea. You could get parents to donate some family friendly magazines and let kids cut out pictures of stuff. You could pick a theme like tell them to pick a color and only cut out things that color, or an object, or natural things like trees, flowers, etc.
Paper "roller coasters"....take a ton of one inch strips of construction paper (just stack em up and slice em on the paper cutter) and have kids glue down one end to a bigger paper and loop the other side in like an arch, then glue down the other side. They can take more strips and loop and glue all around the paper, going over, under, and through the other arches. Older kids can even glue some together to make them longer or fold them and bend them.
Rubbings...this is good for kids of all ages, but especially little kids because it's easy! Get a bunch of flat-ish stuff like combs, spoons, fabrics, etc. and set out a bunch of paper (even school grade paper towels that come on a roll work well for this) with naked crayons.
Melted crayon art. You can keep some crayons on a hot plate and the kids can draw with melted crayons. Also Crayola makes a water color pencil that looks really cool if you dip it in water and then draw with it. My daughter's school also uses this for face painting.
Styrofoam peanut scupltures....I'm sure if you contact a local shipping place they will probably donate a whole bunch of them for free! If you get them just a little bit wet they will stick to each other, no glue needed. You can stack them in all kinds of ways and it looks really cool! All you need is a base of some sort (paper plate maybe) and a tiny bit of water. Be careful not to dip the whole thing in the water or else it will get soggy and gross!
Pony beads are also pretty cheap in bulk and the kids can make jewelry. The older kids can use thread or yarn and the younger kids can use a pipe cleaner and just string them and twist them around a wrist or even a finger!
Grocery sack hats...also, contact a store for donations. Take a paper grocery sack and simply roll the open end down until it's got a nice thick brim and a big puffy top. Have the kids add stickers, glitter, pom-poms, streamers, etc. and it's a festive hat!
Thumbprint animals...I'm sure you've seen the book that shows you how to make thumbprints into little animal pictures, I think it's by Ed Embrly. You can check out the book from the library and just photo copy tons of the pages to spread around the table. You'll need stamp pads (which I'm sure you can rustle up throughout the school) and fine point Sharpies. You can do the prints on paper or on little pieces of recycled wood or something to make necklace pendants. You could even just precut a bunch of stock paper (or maybe even foam if the ink wouldn't smudge) ovals and have yarn available so the kids can make a necklace like that.
There are a ton of crafts you can do with coffee filters. If you just google it you'll come up with a million I'm sure! My daughter has done butterflies with them and they are cute! You can use a clothes pin and some water colors (or markers).
You could also do shaving cream art...just spray it out and let kids draw pictures and write in it. It's a load of fun just a bit messy.
Anyway, it appears I have a lot of ideas...LOL! I hope some of them are useful to you!