Make something using a single color ramp. It doesn't have to be grayscale, it can be other things like blue to orange or purple to green. Musicians, make a song using only one instrament.
Find a piece of art on OGA that isn't usable in a game as-is and finish it. the original piece of art need not be your own. For instance, make something using some concept art, add rigging or textures to 3D mesh, etc. Be sure to respect the original license.
Make a character image that has changable parts, like a sprite that has more than one outfit or a headshot that has different expressions and hairstyles.
Make assets for a small game so that the only thing left to do is program. Entries can be art or sound only. This means the art should have all graphical elements including visual effects and GUI, the sounds should be multiple songs and all sound effects.
