Friction and Food Coloring
You cans easily make things hotter than they were before. Start by rubbing your hands together, and you'll see that they feel hot. That's because the energy you used up moving your hands against the friction of them touching each other turns into heat energy.
Your class can show how molecules get farther apart when they get more energy: first stand still close together on the playground, and then start jumping up and down and thrashing around. After a while, somebody yell, STOP! You will see that you are much farther apart (and much warmer) than you were when you were standing still.
Another way to see this is to fill one clear glass or plastic bowl with cold water and another one with hot water. Add a drop of food coloring to each bowl. Because the molecules of water in the hot bowl are moving faster, they will spread the food coloring faster than the water in the cold bowl (Thanks to CalTech for these two ideas).