If you're using a centerpiece, select an element to incorporate into the napkin ring. For example, for a bridal event, use the flowers from the bouquet for the rings. The easiest napkin ring is a simple bow tied around the napkin. For more pizzazz, tie a ready-made curly ribbon bow around it.
A variation on the bow is to use a wire ribbon bow, then glue on craft-store jewels with white glue. If you are short on space, use a bagel for the napkin ring. This eliminates the need for a bread plate.
For an outdoor party, use twine to tie bunches of herbs to the napkins. A small flower can be added, if desired.
For a garden-themed party, stuff napkins into a clean or new gardening glove.
Make your own napkin rings by cutting lengths from a paper-towel tube into an appropriate width. Reinforce with sticky paper from the craft store, then cover glue on the fabric of your choice. Wrap plastic-covered wire from the craft store into fun, funky shapes. Embellish by gluing on pearls.