An easy and cool way to create scenery is not to create it all! Take your play outdoors and use the beautiful scenic effects created by Mother Nature for your performance. If your play is about fairies of the forest, such as Shakespeare's "A Midsummer Night's Dream," or is a beach-side production of "Treasure Island," plan to stage different scenes within a walkable area and lead your audience from location to location. This scenery idea works best in areas that you will have access to for sufficient rehearsal time in the natural setting.
If you're presenting a Christmas pageant, consider creating a scenic background completely of lights. You could have a decorative criss-cross of strings of lights cover the entire background of your stage. Alternately, have lights strung and connected so that different ones alight at different points in the play. You could, for example, create the skyline of Bethlehem that will light up in one scene and lights forming the shape of a star to light up in another. Musicals that portray the "bright lights of Broadway" or futuristic plays about space would also work well with a light-only scenic design.
Plays with lots of coming and going, confusion or mistaken identity could be staged against a backdrop of a variety of doors of different sizes, shapes and styles. Storybook plays like "Alice in Wonderland," fairy tales or tales from "The Arabian Nights" might benefit from the ability for characters to "magically" appear from a variety of doors. Alternately, consider a backdrop of windows with characters appearing and reappearing in the various openings, but not actually entering onto the stage. This backdrop would work well if your playing space is very shallow.
For kids planning a puppet play, create a special puppet stage from an old refrigerator or sofa box salvaged from the appliance or furniture store. Lay the box on the floor on its side and cut openings in the backside for kids to slide under to operate as puppeteers. You'll need an opening in the top for the puppets to appear through as well. Have the kids decorate the box to suit the puppet play that they'll be presenting--jungle animals and plants for "The Jungle Book"; pumpkins, mice, castles and fairy dust for "Cinderella"; or ghosts, money and falling snow for "A Christmas Carol." If you have the materials and the time create a matching backdrop to hang behind your puppet stage, masking the rest of the space and allowing the audience to easily focus on the performance.