I am in an elementary school and my students favorite things are to take apart old electronics--CPUs, coffee makers, phones, anything..to create with legos, do paper engineering challenges, and use electronic snap circuits.
Recently I purchased small motors and battery holders. We are going to build drawbots first and see what else they can make.
I did not spend a lot of money to get started. Ask for donations such as duct tape, paper towel rolls, craft supplies, old electronics, newspaper, etc. Your student interests will drive your requests and purchases.
Have fun!
Christina