Whatever the theme, you can usually purchase complementary paper plates, colorful streamers and balloons.
Extend the fun to the front yard with a sign announcing the birthday person's big event.
Pick up some posters from a novelty store or museum with vintage airplanes and hang model airplanes from the ceiling. Visit your local Army Navy surplus store and use an old silk parachute to create a "shelter" under which the kids can do their activities.
Visit your local magic and novelty shop for unique novelty magic tricks that can set the scene, like a top hat, black cape, or red velvet covered table top. Decorate the table with cards, beads, disappearing coins, whoopee cushions, or silky scarves tied together in a continuous line.
Create assorted Superhero breastplates from simple materials and polyester padding (your local fabric shop can help) and Superhero shields out of cardboard. Hang them on the wall for decoration, and during the party each child can "earn" his/her plate and shield with good behavior. They make great take-home party favors, too.
Create a mini fire station with yellow hats and yellow ponchos hung along the wall. Transform the kitchen table into a fire truck with a long red table cloth, wheels placed on four corners, a grill at the front, ladders on the sides (made from cardboard spray-painted silver with black) and a large stuffed Dalmatian dog as the centerpiece.
Teens will love anything bold, bright and loud. Music or gaming posters (from local gaming or video rental or media stores), or neon signs are popular.
For little girls, hang arts and craft supplies from the ceiling over the art table (beads, feathers, flowers, paper, plastic jewels). When they are ready for their activity, you can cut what they need from what is hanging.