Of all the activities we have done this has been one of the girls favorites! We ended up playing with them for over 2 weeks! Here is the link we used: http://slsmithphotography.typepad.com/my_weblog/2010/06/lava-lamps-summer-fun.html
All you need is plastic bottles, water, oil, food coloring, and alka seltzer (we ended up going through a few boxes). Start by filling the bottle over half full with oil. Then add water leaving a little room at the top.
Add 10 drops of food coloring.
Break one alka seltzer tab into 4 pieces. And add 1 of those pieces (too much will make the bottle cloudy). Instant lava lamp (without the light)!
Once the bubbles die down a bit you can safely add another piece of alka seltzer.
The science part: alka seltzer releases carbon dioxide (CO2) when mixed with water (which makes the bubbles). The bubbles carry the water drop to the top of the oil where they burst releasing the CO2 into the air and allowing the water to fall back through the oil (since we know water and oil won't mix as water has a higher density)..
Like I said earlier the girls loved these and played with them over and over for two weeks! We just capped them between uses to keep them from spilling.