Lifesize cardboard standup's of Woody, Buzz, or any other of the cast are a sure way to bring life to the party.
Be sure to bring a camera so that you can get a shot of each party participant with the standup.
Once you have your pictures you could either have them printed and given as tardy party favors or simply email them to them or their parents.
Since this is the third movie in the franchise, you may be planning something of a movie marathon, watching the first two, having some kind of party, and then viewing the third film last.
Although we all love the films, it can be rather tiresome to watch that many films without interruption.
That being the case, don't forget to add some fun in between the films with activities, such as the coloring, or trivia games about the films.
Party stores are sure to carry Toy Story 3 themed supplies, so if you want your theme boldly and irrevocably stated use some of those ready made supplies.
Not only do they express your theme for you, but the chances are high that your guests will find those party supplies entertaining in and of themselves with all the pictures of the characters on them.
Lastly, to decorate you might consider using the bold primary colors of red, yellow, and blue, as well as white, as those colors nicely complement the Toy Story theme.
Get streamers, balloons, and and other decorating basic in those colors, and then mix and alternate them to mold them into a congruent whole.
For instance, instead of simply hanging the all four colors of the streamers separate, lay the blue and yellow back to back and the red and white back to back.
Attach the end at your starting point, twist as usual, and attach at your desired ending point.
This gives your streamers a more dynamic appearance and allows you to use several colors without becoming chaotic.