Install a dropped ceiling. Insert a second ceiling below the ductwork to cover it completely. A dropped ceiling eliminates the sight of the pipes without causing any damage to them. This tactic is rather expensive, though, and may lower the value of the property.
Box in the ductwork. Assemble wooden boxes around the ducts to encase them, hiding them from view. Paint the wood the same color as the ceiling. This option is cheaper than the dropped ceiling but will still leave the outline of the ducts across the ceiling.
Paint the ductwork the same color as the ceiling. Painting will not hide them, but it will help them blend into the background. Use a metal paint since most ductwork is a metallic silver color.
Contrast the ducts with the ceiling by painting them a loud color. Instead of trying to hide the ducts, draw attention to them by painting them bright blue, orange or yellow. Use a suitable paint so it adheres to the galvanized metal.
Attach strings of LED lights around the ducts. Under normal circumstances nothing should be laid over ductwork, but strings of Christmas lights could create ambiance and gently light the room. You may need extension cables to get the light cables to a power outlet.
Change the decoration on the ducts depending on the season. For instance use Christmas lights and tinsel during the holidays, cobwebs and plastic bats during Halloween and flower garlands during the summer.