There are so many patterns and decorations that you can do with the #18 tip.
I'm going to put up videos I've found on Youtube that have come in useful for me. There really is so much you can find on that site! So to save you having to spend time searching for videos I'll add them here. The first video I'm going to add is to show you how to pipe zig zags and rope onto cakes or cupcakes.
It's really quite easy to do and looks great. I just used frosting from a tub this time (I'd run out of butter to make my own).