One of my favorite things to do to bring my brain down from its creative highs, is to browse the app marketplace. Carefully deciding on what odd photo-manipulation app they have come up with the month, I usually choose one that will entertain me. What I like to do is throw photos of my originals paintings into these apps and see what it does. It it like I get to collaborate with a robot, or math or nerds.... Not sure. I included some of my favorite apps that I think are damn fun creatively. Love ProCreate. Really awesome drawing program for ipad. Layers, transparency, total brush control and unlimited other options. Can even save the file in full psd format.
iDraw is fresh. Closest thing I have found to have a program that can act like Adobe illustrator on the iPad. Vector goodness. Not sure if it has a live trace ability... But I am still playing around in it.
SpacePaint is fun. It overlays all these space, lightning fire graphics over background image you choose. You can then go in and get technical and erase out parts of the graphics, to make it look like it is real. Awesome and my kids love it.