Jolly ranchers into lollipops. Great way to show kids physical changes of matter. Also a yummy summer project.
Make these simple sensory bottles as a hands-on manipulative to talk about the weather. Great for circle time!
Stick an acorn in some water, grow an oak tree. Sooo cute.
"HOW TO: Grow an Avocado Tree from an Avocado Pit" -- an illustrated step-by-step guide
How to Grow Avocado Trees from Seeds trying this today with the guacamole left overs! (We started one of these and it is growing roots after about 10 days. I'll try to update as it does more. ~J)
Actually realy cleaver and useful for vegetables like carrots so you can see when to harvest. And also it looks freaking cool! :D
How to grow a mango from the pit -- with links to germinating orange and grape seeds, planting strawberries indoors, and growing a tamarillo tree in a pot -- wikihow