Teach little ones an environmental lesson by showing them how to plant a seed and care for a plant. Give each student a small flower pot or Styrofoam cup and let them decorate it. Help them fill it with soil and plant a few seeds inside, then water it carefully. Explain to them about how that particular plant needs regular watering and light (see the instructions on the seed packet), and help them care for their plants in the classroom over the next few weeks. As their plants blossom and grow, they will learn about caring for the environment and sustaining plant life. You can also show them the life cycle of a plant through illustrations or books, so they can understand what is happening beneath the soil.
Build on your potted plant lesson by planting a small ruler in the cup next to the plant. You can also just use a strip of card stock with centimeter or inch markings on it, if a ruler is too large for your cup or flower pot. Have students mark off the plant's growth every week and keep track. This will teach them measurement skills as well as scientific observation.
You can even have experiments to see if the plant grows more or less in one area of the classroom or another, or if it grows better with fertilizer or without, or with more or less watering.
Use leftover plant seeds to create a crafty art project. Allow students to draw a picture outline on paper and then paint it with glue. Sprinkle a variety of seeds onto the paper to make a collage. They can create a picture of their own plant or create a flower out of their imagination. Either way, they will enjoy the creative art project, and it will give them a way to pass the time as they wait for their flowers to grow.