Mobile
Mobiles make ideal solar system models since you can suspend each planet in its rotational plane on a mobile, and it can turn on a central axis. Hang the mobile from a single string that passes through a disc or disc-shaped wire armatures to suspend the Earth and the other planets. The ball used to model the sun should be the heaviest to weight the model. You can model the other planets with something light such as sphere-shaped polystyrene foam balls in varying sizes, which is available in arts and crafts stores, focusing only on their relative sizes. If the planets need to be balanced to hang correctly, use small lead fishing weights to adjust them.
On a Disc
Any disc is ideal to model our star -- the sun -- and our planets, but a sports disc is an especially fun place for the model. Because a sports disc spins when it's thrown, it naturally illustrates the concepts present in the model. Use a fine-tipped permanent marker to draw each diameter, showing the orbit of the various planets along with the outline of the Earth and the sun. Then color each with different-colored markers and label them. A benefit of this fun model is that you can also play with the model and observe it as it spins, so it's a convenient place to summarize planetary information.
Painted on a Wall
A mural is an ideal place for a model of galactic information. Particularly good for a kid's bedroom wall, you can paint the mural as a two-dimensional representation, or a three-dimensional representation showing the elliptical perspective of foreshortening in the model. With a wall installation like this, you have plenty of room to write or decoupage information about the Earth and planets onto the wall. You could even transfer a solar calendar off to the side of the main image for reference. Use an opaque projector to transfer a small image to the wall for this kind of model.
Focus on the Earth
The model doesn't have to convey the same amount of information about each planet. In any of these models, you can highlight the Earth, sun and their relationship. For instance, you can represent each planet graphically by its relative size only. Or, you can accompany the planets by only a single datapoint like their distance from the sun or their diameter, and you can accompany the sun and Earth with additional information. Even if the model is a mobile, you can model corresponding information and place it on the wall, for example, in the same manner that an aviation museum would place information about an aircraft suspended from the ceiling or an installation on a placard so that it clearly relates to the model.