Craft Stick Catapult - I made 4 of these in 5 minutes and they work great! You need craft sticks, 4 rubber bands, hot glue or tape, and a bottle cap (I cut apart an egg carton because I didn't have any bottle caps).
Looking for a new toddler activity? This one is fun, easy and engages fine motor skills - plus it's just cool to make your own musical instruments! Chelsea from /twotwentyone/ shares the step-by-step (Step Class Fun)
Science -- Weather: Fill the cup with water. Put shaving cream on top for a cloud. Explain that when clouds get really heavy with water, it rains! Then put blue food coloring on top of the cloud, and watch it rain. Air and Weather unit
"Volcano Song" would be a good opening activity for the volcano lesson. It highlights the important aspects of volcanoes (such as lava and magma) in a way that the students will learn from and remember.