Start a planting project at school -- a class garden is a good beginning activity. Grow vegetables and native plants that do not need too much watering. The students tend the garden themselves and can sell their produce to the community to raise money for environmental charities. Children can also plant native plants and trees at local parks or community centers. They can research which types of plants and trees grow best in their climate.
Clean up a local park, beach or outside a community building. Ask the local government to donate orange vests, gloves and trash bags. The fifth graders can meet during school or on a weekend to clean up trash and unwanted debris. It is instills the notion that they are the protectors of the environment. Plan other cleanups at different locations and invite other schools to participate.
Repurpose an old T-shirt into a drawstring bag to help the environment. Turn the T-shirt inside out and cut across the top of the T-shirt just below the armpit level to remove the top completely. Sew across the cut edge, which becomes the bottom, to seal the bag. Turn the T-shirt right side out with the newly sewn bottom facing down and cut a small hole into one side of the bag's top hem (the original bottom hem of the T-shirt) between the hem and the edge of the shirt, so that the space between becomes a tube. Thread a clothesline cord through the hole in the hem so that it becomes a drawstring for the opening of the bag.
Fifth graders can start a recycling challenge at their school. Ask a local recycling company to drop off recycling dumpsters for each class or grade level at the school. Start a contest to see which class or grade level can collect the most recycling over a two-week period. Students and parents bring all their household recycling to their class or grade level's recycling dumpster. The winners receive a pizza lunch.