Almost anything you like! Paper, plastic, fabric, metal, clay, wood, natural elements, glitters and powders, and any combination of these elements work well. Things that contain moisture should be avoided, for example, fresh flowers and leaves. The water will react with the resin, turning the leaves and petals brown. Dried flowers and leaves work beautifully. There are some things that don't look so great - iridescent feathers will lose their shine, locks of hair may clump together and look like a claw.