If you live in a historic building or house, research the various styles of stencils that were popular during the time your structure was built. Books such as "American Wall Stenciling, 1790--1840" by Ann Eckert Brown follow the history of stencils in America from some of their earliest examples to just before the Arts and Crafts Movement. Books like "Stenciling the Arts & Crafts Home" by Amy Miller have information about stencils for the Arts and Craft period. The books show how stencils were used in various rooms.
In the dining room of a colonial house or building, consider food-themed stencils such as pineapples (which also symbolized the height of hospitality) alternating with wreaths and leaves or flowers. In a child's room, have twining vines and flowers done in the style of your abode's era, and even add some twists of your own. Generally speaking, in smaller rooms, use smaller, more delicate stencils; in larger ones, use larger and more intricate ones.
Draw designs for your stencils on cardboard or vinyl sheets and cut them out with an X-acto knife. Trace portions of designs from books onto vinyl sheets and make those into stencils as well. Create freehand stencils that incorporate balls or toys for a child's room, or more period-appropriate stencils like bugles, horses, and drums, or arrangements in flowers or fanciful creatures.