If you're having a summer barbecue, for example, use a beach pale as an ice bucket and a beach towel as a tablecloth. You can even use color as your theme and have several elements of your table (tablecloth, napkins, glasses, bowls, etc) be a different shade of that color. The easiest way to come up with a theme is to think about what you already have on hand to decorate, and go from there. The more you have on hand, the less you have to buy, the better off you'll be.