I built the only working volcano in my class once, lava and everything. Red colored plaster of paris hardened in he bottom of a can (volcano mouth) then 1 cup sugar 1cup salt peeter sp? Mix, light a wooden match and stick it right in the sugar, smoke lame and red lava.
Another fun one is coffee mate, don't know what science it is but put some in your had a light a match own low and let the coffee mate stream from your hand and stand back
Potatos running a light bulb
Magnets are always fun