Put water in a bowl, drop random colors of nail polish in and swirl with toothpick. put petroleum jelly on your fingers so the polish only sticks on nails not your fingers. Stick fingers in bowl, nail polish should float on top of water.
French tips using scotch tape. I have done it with circle stickers but this might be nice for straight cut nails. This person wore the base coat for a day or two and then did the french tips to extend out her manicure.
Multiple stripes of different colors of crackle polish with the top layer being a full coat of black crackle. The same effect could be achieved by using regular polishes to do stripes on the nail then topping off with the black crackle.