Make a potion! Nothing looks more like a potion that a fun chemical reaction. This one done with kool-aid and baking soda is easy and fun!
If you want to impress students with a simmering cauldron, I love the way a fog bubble looks right after it pops.
Creepy crawly spiders are also great fun to learn about around Halloween. Spiders have such interesting animal adaptations.
And of course pumpkins are appropriate for both the little kids (kinder and first) and the bigger kids (3rd grade life cycles)
So many great ideas for Halloween and fall Science! Here I am as Ms. Frizzle for Halloween! Who's ready for a field trip?