Here's a brightly colored science experiment that not only looks cool, but allows students to develop their own understanding of density! I used this experiment for a freshman Physical Science class, but it could be adapted for many ages and situations. It's also an incredibly cheap experiment that can be done with virtually no lab equipment if necessary.

Objectives:

1. Explain density using examples.
2. Identify and use laboratory equipment.

Other potential objectives:
1. Measure mass.
2. Define mass.
3. Measure volume.
4. Define volume.
5. Calculate density.
6. Explain density conceptually.
7. Compare the densities of different materials.
8. Design an experiment to test a hypothesis.

Step 1: Materials

You can make this experiment anything you like. I used it very early in the year. Students had measured and timed things for the velocity unit, but I was still teaching measurement and introducing them to the lab. I usually included every skill they could possibly practice.

Required:
Pipets (Can be purchased online from all kinds of vendors. Example: http://www.flinnsci.com/store/Scripts/prodView.asp?idproduct=14269 About $25 for 400)

You could substitute syringes, turkey basters, or some other device that would allow you to add the liquid to the bottom of a tube but pipets are so useful you might as well buy some if you don't have them.

Useful (but could substitute):

250mL beakers (4-6 of these)
Food coloring
Salt (about 36g = 6 Tablespoons)
Stirring rod
Measuring spoons
Test tube

Substitutions:
Cups or bowls instead of beakers
Natural dyes instead of food coloring (Think beets for pink etc.)
Sugar instead of salt (This is a little messier and if not well cleaned up more likely to be a problem but works just as well.)
Spoons instead of stirring rods.
Any kind of spoon that you use could be substituted for measuring spoons. You just need to know that you're adding a relatively standard amount more for each additional color.

Optional:
Electronic Scale or Triple Beam Balance for extension
Test tube rack
Test tube drying rack
Test tube brush (for cleaning)

These optional ones are more about teaching them about these pieces of laboratory equipment.

Step 2: Procedure

I usually demonstrate the procedure for students and then have them write a procedure and/or draw pictures of each step. Especially since I did it early in the year, I wanted them to practice writing their own procedures and then critically analyzing what could be improved to make the procedure understandable for someone else. You could also do this as a whole class to model what the process of writing a procedure "looks like" and then have them write their own later in the year.

I usually rushed and did an imperfect job during the demonstration so they could "figure things out" but had the general idea of how to do it. I'd start by demoing really quickly adding the first and second layers to show that they would just mix and then do one "right" to show the value of doing it slowly and gently. Every student was able to do it by the end of class and it was a great way to start off their science experience with something that was a little challenging at first, but then successful.

Make Salt Water Solutions:

1. Add 200mL of water to a 250mL beaker.

2. Add 1 Tablespoon of salt to the water. (Technically you should probably add the salt first and then the water.)

3. Add 4 drops of food coloring. (I let them add as much food coloring as they want and whatever creative colors they want to. Buy food coloring at the dollar store and it is really cheap.)

4. Stir with a stirring rod.

5. Repeat, increasing the amount of salt added by 1 Tablespoon for each successive solution.

6. Have one beaker of water with no salt for the least dense condition.

Creating Layers:
1. Choose a volume of each color to add to the test tube. (2mL works well for small test tubes. 4 mL works well for larger ones. I'd try this out once before hand with your supplies. It's easier to only have to add one pipet of liquid.)

2. Add 4mL of the least dense solution any way that you want.

3. Add 4mL of the second least dense solution by very gently inserting the pipet tip into the liquid and to the very bottom of the tube.

4. Very slowly and gently release the solution. A new layer should form. Keep the bulb of the pipet squeezed. Very slowly and gently remove the pipet from the test tube.

5. Repeat with the remaining colors.

Step 3: Additional Ideas and Resources

Extension/Modification Ideas:

Use a triple beam balance of electronic scale to teach or practice that skill
Use hot and cold water instead.
Compare salt and fresh water and connect with the environment.
Have older students teach this to younger students.
Use other liquids to make density columns. (Water, oil, alcohol, etc.)
Make a much larger one as part of a science night.
Let students try making different density salt solutions and try to figure out which ones work the best and how that relates to density.

Other Density Demos:

Egg in salt water or tap water
Ice in water or rubbing alcohol

Attachment:
I attached a Word document that is a template for a guided lab report that you feel free to modify and use.

Other Ideas and Resources:
These are just some ideas and alternate extensions.

Some additional references will many different versions of this same idea:
DENSITY: SALTWATER COLUMN LAB TEACHER PREPARATION
Explains a different way to prepare solutions and some "discussion" questions as a worksheet for students

Liquid Rainbow
Written from the standpoint of ocean science for elementary school. Contains a lot of "teacher lingo" (e.g. key concepts, big idea, objectives, etc.)

GEMS: Discovering Density book
This book costs $18, but I think is where I first saw the idea for this lab. The handouts are free online in Spanish. So that might make this a cool activity to do with students learning Spanish too!

Steve Spangler 7-Layer Density Column
This link provides a video showing a different density experiment with different types of liquids to see how their relative densities are different. They do use the word "heavier" instead of density. He uses a slightly different way of layering.

NOAA Lesson Plan: Hot, Cold, Fresh and Salty
Another Earth Science/Oceans connection. This lesson plan gives ideas about having students compare hot and cold water and fresh and salty water. These are extensions that would be great to add to a simpler lesson like the one I posted.

Step 4: Real World Connection

In research labs, human blood is carefully pipetted to form a layer on top of a substance called Ficoll-Paque (made by GE). When this tube is centrifuged, the red blood cells (the most dense) go to the bottom, the Ficoll is (the next most dense), then the white blood cells, and finally the plasma (the least dense).

Labs use this technique to isolate different parts of the blood. For instance, if you want to isolate the white blood cells, you can remove the plasma layer (yellow) and then gently extract the thin cloudy white layer that contains the white blood cells.

Wikipedia's Ficoll-Paque Article
This page describes the basics of Ficoll

Ficoll-Paque PLUS Manufacturer's Description
A short paragraph from the manufactures that uses the word density multiple times

Ficoll-Paque Instructional Video
This video shows the layering of blood on top of the Ficoll very slowly and gently and the final layers that form

Step 5: Graphic Organizer Word Document

Someone let me know the Word document does not open currently. I tried uploading another version but it also says forbidden. Message me and I can try to send it to you.