One of the most popular solar system projects is building a model version of the solar system. One way of doing this is to paint Styrofoam balls to represent the planets, then attach them with wire to one large Styrofoam ball in the middle, representing the sun. Another way is to create a model to scale on a football field, using an orange as the sun and calculating the distance of the other planets.
A cardboard sun hole is a simple way to demonstrate the power of the sun's rays. You'll need a thin piece of cardboard, a piece of paper and a pen. Use the pen to punch a hole in the cardboard, then hold the cardboard over the piece of paper in direct sunlight. You will notice a small white circle on the paper--an upside-down image of the sun. Move the cardboard forward or back to focus the circle. If a cloud passes over the sun, its shadow will cross the page in the opposite direction.
This is a fun research project to help third graders learn about the planets. Pretend that you are going to take a vacation, only to Mars instead of Hawaii or Florida. Have the students imagine what it would be like to be there, what they would see and do, what they would need to bring with them, and so on. Research facts on the planet Mars, then have the children write a story or travelogue about their trip.
There are a number of fun solar system assembly kits available online at amazon.com and other sellers. One of these kits is the glow-in-the-dark "Mega Cosmos" kit, which contains glow-in-the dark stars, as well as nine planets for the solar system. This and other kits can be assembled by third graders, either alone or with the help of an adult. Solar system kits come in two- and three-dimensional versions. There are also star ceiling kits, which have glow-in-the-dark star stickers to put on the ceiling.