A simple square cushion using two contrasting fabrics, based on the tutorial from MildlyCrafty.com, is chic and modern. The final pillow measures approximately 20 inches square, but you can make it in any size and fabric to fit your desired look. If you care to whip up a few of them for use on the patio, use waterproof or wipeable fabric on at least one side.
This tutorial from Martha Stewart shows you how to turn a plain cushion cover into something special using petals cut from ultrasuede and sewn in a flower shape across one corner of the cushion cover. Ultrasuede is different from normal suede; it doesn't fray, so all you need to do is snip around the template and attach them. There is no machine sewing required in this tutorial, only basic hand sewing, so there is no initial outlay for a sewing machine if you don't already own one.
If you have a little more time to spend on your cushion project, try out an applique. The technique involves sticking or sewing fabric cutouts to another piece of material. The tutorial by CraftyHabit takes you step by step through the technique and you'll end up with some covers that look lovely arranged on a couch or on a bed.
If you are looking for a quick project that can be made in no time, the Momfetti.com cushion is for you. The tutorial is simple to follow and shows how to create a basic cushion cover without all the trimmings. Make these for a quick "going away to college" present or for an extra layer of cushions behind the fancy ones on the sofa.