There are many ways to personalize your phone case. Oftentimes, people use stickers or decals of items they enjoy as a way of technological self expression. However, if you would like to get really creative, you could always paint it. Some people paint landscapes or animals, but if you have a significant other and are confident enough in your artwork, you could always paint a picture of that person; the gesture would likely not go unappreciated.
Beyond stickers or painting, there are other options for customizing your case. You could get small plastic gems at a craft store and attach them to the case yourself, but be warned. Doing this is often an issue, as the gems tend not to stay attached for a very long time.
Another option you could try is to buy some craft paper with designs on it and cut out the shape of your phone case. Leave a little extra around the edges to make it easier to manage; the larger size helps if you accidentally make a mistake, and you can always cut the extra off when you're finished. Then simply glue the paper onto the case. Be careful to use a glue stick instead of a tube, as the tube glue can cause annoying textural changes and an overall lumpy appearance.