I like to do patterns, from the easy ones (checkerboard, diagonals) to the difficult (cube in a cube in a cube)*
*I'll explain that one:
Starting with a solved cube, looking at three sides... if you could just take a 2x2 cube and rotate it, now each side has an "L" of one color, and a 2X2 that's the color of an adjacent side. Now, if you could just take the corner piece in that 2x2 "cube in a cube" and rotate it again (the same direction) you'd have each of the three colors on each side, and it looks like a single cube rotated in a 2x2 cube rotated in a 3x3 cube. But of course, you can't just take a 2x2 and rotate it, you have to solve it as a whole.