One way to show the cohesive properties of water is to show how things float for some time on water. A good craft to use for this project is the newspaper man. Cut a man out of newspaper and lay it on a pie plate or soup plate filled with water. The man will float for some time. Try the experiment several times with new newspaper men and time each one to see how long they float. For a compare and contrast activity, have two plates of water, one plain and one with a drop of liquid dish soap in it. See how fast a newspaper man will sink when put on the plate with the dish soap and the one without. Since the dish soap breaks the bond between water molecules, that man will sink faster.
Another interesting activity that illustrates water cohesion is the Super Penny craft. Using an eye dropper, have children demonstrate the super powers of a penny -- see how much water it can handle. Put water on the penny one drop at a time, using the eye dropper until water finally spills over the penny's edge. The penny will hold much more water than the children will think it could. Cohesion will let the water mound up. To compare this to the number of drops when there is little cohesion, rub a drop of detergent on the penny first, then add drops of water.
The cohesive properties of water make this interesting craft work. All it takes is poking a row of holes 1/2 centimeter apart at the bottom of a jug, cup or tin can. Put the container under a tap of running water and allow the water to keep running so that the container is nearly full all the time. The water will run out the holes in the bottom of the container in separate streams. Next, pinch the water streams together. When you take away your hand, cohesion will keep the streams of water together.
An interesting demonstration of cohesion and adhesion in water is a string bridge. Wet a string, piece of yarn, wire or twine and suspend it between a dripping faucet and a cup or between two cups. Let the water drip onto one end of the string, and watch how cohesion lets the drops stick together and slide down the string into the other cup. This craft is best if the water is poured in drips, not a steady stream.