Here are some facts about density.
This is how to build a 12 layer density tower.
Here's some Bill Nye air pressure demos.
Wind and air pressure explained.
Bill Nye explains ocean currents.
Here's a cool convection demo.
Here's another cool convection demo.
Here's a density tube made by Nevaeh.  It's got 5 layers. (Alcohol, vegetable oil, dish soap, milk and syrup.)
Here's a density tube made by Curtis C.  It's got 5 layers. It starts with honey, then maple syrup, water, coffee creamer and finally olive oil.  Yum! 
Lexie S made a pair of density tubes.  The left one has honey, dish soap, water, vegetable oil and rubbing alcohol.  The right has honey, maple syrup, dish soap, vegetable oil and rubbing alcohol.   
Kyra Z's jar is made from chocolate syrup, maple syrup, milk, water, canola oil and rubbing alcohol.  At least half of that is yummy.
Maddy B's density tube is
made from honey, corn syrup, chocolate syrup, maple syrup, dish soap, cold water, warm water, vegetable oil, rubbing alcohol and baby oil.  Geesh, that's a lot of stuff!
Caitlyn J made hers with water, apple cider vinegar and oil.
Ryan N made a density jar with cooking oil and milk.  Two tastes that taste great together.  
Andreja S made one honey, milk, dish soap, colored water and canola oil.  Wow!