My mother has a round indoor dining table for which she's always made elastic-edged tablecloths. They're casual and always stay put, which has us thinking of how very well they would translate to an outdoor
To make your own, simply over-cut a round piece of cloth by about 4" on all sides (for a 36 diameter table, for instance, cut a 44" diameter piece of cloth). Sew an elastic band into the hem and stretch the cloth over the tabletop for the simplest of versions. Add a ruffle around the edges (as in the photo above) for a bit of an overhang. We could see doing this in all sorts of fun colors and prints for different summertime occasions.
Related: Summer Table Inspiration from Chelsea Fuss
(Image: from a Ruffled Burlap Tablecloth tutorial by Tatertots & Jello)