So, since someone said "rainbows" when I asked for suggestions, I decided to combine that with nyan cat. And this design took me two days actually, because the different parts had to dry before I could continue, and I went to do other things in the meantime! :P
I started with this background, with a light blue fading into a dark blue. A friend showed me the technique. I used a sponge and painted a part of it light blue, and the other dark. Then I "sponged" it onto my nails.
Then I painted the nyan cat on my thumb, with the start of the rainbow after it.
And finally I painted rainbows and stars on the rest of my fingers. Hope you enjoyed!