This idea came from http://slsmithphotography.typepad.com.
We reused the bottles from water bottle bowling, to make lava lamps. Then we got vegetable oil, water, food coloring and alka seltzer.
Fill your bottles a little more then half full of vegetable oil, fill the rest of the bottle, leaving about an inch of space, with water. Add food coloring, to make desired color. Again, food coloring was a huge hit. Add half of an alka seltzer pill at a time, and watch the "lava"!
The good news is, the bottles can be stored and saved for more use later. The kids also liked watching how the water and oil didn't mix.