My son just celebrated a birthday and when asked what kind of cake he wanted he didn’t come up with an answer until the night before the big event.  His answer was that he wanted a Star Wars cake…. no other details were given. 
I decided to make a light-sabre.  The reason I’m sharing this with you is that is was so easy it was scary, it looked pretty cool, and you can adapt the idea.  Here’s a picture:
I simply made a 9x14 sized cake, cut it into long strips, pieced it together on a big tray and iced it.  What made the kids love it is the addition of glow sticks along the sides.  I already have an order for another one in November.  (I’m not sure why it looks so crooked in the picture?!?!)
If a light-sabre is not something your birthday person would be into, you could transfer the same idea to a princess wand, fairy wand, change the shape of the cake and you could do a lava lamp, a glowing retro diner clock, any neon sign you want to reproduce in sugar….. you get the idea! 
Any other suggestions I’ve missed?