Building a simple stand from scrap wood for a flat mirror allows you to reflect the blue sky. Any mirror will work, but plastic mirrors last longer in outdoor conditions and don't weigh as much. Stick to inexpensive mirrors because the moisture and sun exposure will cause the frame to warp after a few seasons. Position the mirror so that it reflects light into nearby trees or bushes instead of into a neighbor's house or the street.
Hang a framed mirror on a fence to create the illusion of a window to another garden. Paint the frame around the mirror to match the fence. Planting trailing or vining plants on the fence partially obscures the mirror, helping it blend into the garden instead of standing out starkly. Weather-proof the frame with wood sealant, or use a plastic or metal framed window to ensure your decoration lasts for a few years.
Mirrored surfaces in the garden don't have to be flat and framed. A simple gazing ball reflects the garden in different perspectives because of the curve. Instead of putting a mirrored ball on a pedestal, though, roll it into a flower bed or hang it from a tree. Colored reflective gazing balls add more light to the garden without being as distracting as a framed mirror.
Broken mirrors lend to a garden decoration project with the help of concrete. Like glass or stone tiles, press bits of mirror into the top of wet mortar or concrete to create mirrored stepping stones. Or glue mirror pieces to pre-cast concrete objects, such as garden tables or bird baths. Mix the mirrored pieces with other tiles to prevent your decoration from looking like a disco ball.