My cousin is a party decorator and she found some tiny lights on Amazon used for lighting up the inside of balloons, and for table decorations.
The size of a small marble, bet they'd look cool as bellybutton lights.  

They are bright and come in colors, color changing, flashing ect.  Google amazon balloon lights and you'll see them, don't know if this link
will work
balloon lights

anyway they are inexpensive, i have a few if any ladies want to try