Smaller vase inside larger to decorate vase. That'd be super easy for centerpiece. Floating candle in the middle. Could do it with lots of other things besides lemon slices. Beads, pearls, flower petals, fabric, ribbon... Possibilities are endless!
DIY~ How to make a peony topiary- Great tutorial. This lady used these topiaries at her wedding. There's also a matching wedding bouquet on this site that is made of pink peonies and green hypericum berries that is absolutely gorgeous.