I have been teaching fifth grade science for eight years. These are some of my favorite activities:
Some science activities
Force and Motion:
Rockets - NASA has an excellent pamphlet titled "Rockets" with lots of actities in it.
Mousetrap cars/reaction cars
Make a model cell out of jello as cytoplasm with candies and fruits as structures in the cell.
Make a model solar system. There are versions that use a standard roll of toilet paper to show relative distances between the planets. A model can be set up on a large playground using a golf ball as the sun. A solar system in motion can be set up on a playground with students as planets and moons.
Light and energy:
Teach that energy can be changed from one form into another. Demonstrate transformations such as chemical energy by burning a match to change into thermal and light energy or a baking soda - vinegar reaction to show chemical energy changing into mechanical energy. A flashlight shows chemical energy turning into electrical energy and then into light energy. A photoelectric cell changes light energy into electrical energy.
One of my fellow teachers gives students a soup can and a collection of other materials like rags, styrofoam, aluminum foil, etc. He has them make a "cooler" into which they put an ice cube. The students compete to see whose ice cube takes the longest to melt into water.
Chemical and Physical Properties:
Have students observe and measure properties such as magnetism, density, color, state, and electrical conductivity. Have them test a collection of items for these properties and then use the results to make a classification system for those items. A fun chemical and physical properties activity is the "Powder Mystery". Search for that title online and you will find lots of variations of it.
Feel free to email me if you want or need more specifics on any of these things, or if I can give you more ideas on specific topics or concepts.
I love teaching science. I hope you will, too. Good luck,