One basic theme idea is to decorate your entire classroom like the solar system for a few weeks. Have the planets going around your walls, put up large black tarps or sheets of construction paper and have kids color in and decorate planets using pictures of the solar system as guides. You could even purchase foam balls in various sizes from a local crafts store, paint them to resemble the planets and hang them in the classroom. Cut out star shapes and glue stars to the ceiling to represent different constellations as well.
Another idea is to decorate the classroom like a specific planet. Do a new planet each month to stretch out your lesson plan over the school year. Research planets with second graders to learn about what colors are found on them, which planets they are next to and what the environment is like. For example, for Mars you could have yellow and off-white rocks decorating shelves and on desks to represent the rocky terrain, and hang two foam craft balls in the classroom to represent Mars' two moons.
Put on a show based on the solar system for an interactive twist on the lesson. Assign each child a planet to play the role of and also assign moons, constellations and comets if you have a larger class. Kids can make costumes to fit their planet. For example, a child portraying Neptune could wear a baby blue sweatshirt with white lines whirled around it to represent the look of Neptune. A student representing Saturn could have wires around her waist to represent Saturn's rings. Create a short play by having the kids stand in the order of the solar system and each recite interesting facts about their planet, star or comet. Put the play on for other classes or hold the play after school for friends and family.
Add a competitive aspect to your solar system lesson by holding a competition. Have students make models or representations of the solar system at home and allow them to use whatever artistic methods they like. For example, kids could use various fruits and vegetables to represent the planets, make dioramas, use clay and paints, draw pictures or use paper mache. Students can bring in their pieces and show them off to other students, then vote on things such as "Most Accurate," "Most Creative," or "Best Overall."