What a fun thing to do...really 'tasting' life!
So, as a preschool teacher myself, I tend to collect things for projects. Here are some you might consider:
Pressed autumn leaves. This is a great use for old phone books. Collect the leaves, wipe clean, press. You can do this with flowers too.
Some home and garden magazines. I like National Geographic, too, however, I do tend to purge those of upsetting images before letting the kids peruse them for collage stuff.
I'm a saver: that is, I collect up good ribbon, wrapping ribbon/raffia that's in good shape. I also save tea tins and sturdy glass jars with their lids (for bug houses). If you have baby food jars with lids, those are also good to save a bag or so of. I used them for wet-on-wet watercolor activities and have all of my son's tempera paints ready to go in the larger-sized baby food jars.
The baby food jars and tea tins are great for storing the extras, by the way.
We also have a nature table at our home (I've got an old printer's type tray set up on its side for small treasures) full of shells, rocks, feathers, pinecones of all sizes, seed pods and other interesting tidbits which will work in a pinch if there's a need at school. Dried seed pods can be really neat for artwork.
Buttons. A container of errant buttons is great, especially as manipulatives for practicing counting or for more advanced beading (big needle and embroidery floss.... good fine motor activity for 4yo +.) Buttons and popcorn, seeds can be glued onto just about anything.
What Amanda listed below is also helpful. I would add a hole punch and stapler to the list. What I listed may be more than you need (I didn't go into all the other stuff I've collected-- it would just make your head swim!). Have fun with your little one!