I think this a great time to introduce children to the worlds natural resources, considering how much they love to play outside. I definitely think you should utilize the various landscapes around the school. Go on a nature walk with your kids, and have them observe, illustrate, and collect samples of nature through out the walk. You can use sand from the sandbox, to the rocks that make up the gravel, or dirt vs. clay that you can find at schools ( or you can bring in different samples). Have students bring back various samples, of different textures and materials to classify into the appropriate natural resource category. Having the students draw and label what they find helps to integrate small moments that will help then retain the information. Students should also share the similarities and differences among their illustrations. That way when they think of natural resources, they will think back to the nature walk and everyone's different illustrations.
Whatever you choose, you're on the right track of having them engaged and getting dirty! Keep it up! Best of luck!