My grandpa loves trains so I usually make something train related (like this Soo Line fleece blanket) for him when I give him gifts.
This year as I was trying to figure out what to make him for his birthday I was inspired by all of the links on the internet (like here and here) about customizing ceramics by drawing on them with a black Sharpie marker and then baking them in the oven to set the design. I thought this would be a perfect technique to make a train coffee cup for my grandpa. 
I brainstormed about what design I should make and decided that a cartoon aerial view of train tracks would be cute. I started by drawing a set of thin parallel lines for the tracks on the mug. At two spots I stopped and drew a series of rectangles to represent the top view of the train. I went back with a thicker black Sharpie and drew in the railroad ties over the tracks. I wanted a little color so I filled in the train cars with red, blue, green and aqua Sharpies (with the caboose being red of course!). I also added some puffy smoke coming from the train engine.
The cup still looked a little plain to me so I drew in stripes along the bottom part of the cup with the various colored Sharpies. I made a random pattern and alternated thin and thick lines.
Once I was happy with the design I popped the mug in the oven at 350° F for thirty minutes to set the design and then let it cool. The cup seems to hold up well to hand washing, but I've read that the dishwasher can cause the design to fade.
Since I free handed everything there is an inconsistent wave to some of my lines, but I like the look because it shows that it was a handmade gift. Hopefully Grandpa likes it!
Have you tried customizing anything with a Sharpie? What did you make?