You got any super-heroines you like? It's not super revealing, it's sexy (especially with the tighter clothing), and if he likes comic books or is generally into that, it's a great way to start up a conversation. Try to pick one you already like, but isn't super popular (i.e. Wonder Woman or Cat Woman), so he knows you put some thought into it. Do some research on the subject if you must. Try to create your own costume (body suit, tights, and a little bit of imagination would do the trick for most costumes). Storm or Jean Grey from the X-men would be cool, as well as Jubilee or Rogue (my costume). You could even do non-hero characters, like Mary Jane from Spiderman.

Characters for video games and action cartoons are fun, too. A lot of them wear fairly easy-to-find clothing, and have a signature look. Suggestions on my part would be Chrono Trigger, Legend of Zelda, Digimon, Pokemon, Lara Croft, Final Fantasy, King of the Hill (Luanne), etc. Go for something that isn't going to be difficult to recreate, with your time limit.

Another idea (if you aren't into the tight-clothing and capes, or don't want to do a lot of research/don't have a character you resemble and like) would be a gypsy or a pirate, both of which are easy, and use things you can wear otherwise. Try finding a corset (or even a fake corset from a halloween store), and/or a peasant top. A long, flowing skirt or sarong(s) would make a good bottom. You could even wear tighter, brown pants, or a shorter skirt if you want to show off your legs. Just put a lot of color and glam into the gypsy costume. Older-looking metal jewelry (legit or fake), dark eye make-up, removable/drawn on tattoos (stick to something within the theme, and without much color), a sword/knife (for the pirate), gold coins, an instrument (for the gypsy, preferably something solid or that you can play), scarves, and some brown or black boots are all great details for these looks, and if you don't already have the means for them, you can easily get them.

You could also try a musician you especially like that has a distinct style. You don't have to go all Lady Gaga on him, but there are other musicians that are pretty stylin'.

A pin-up girl would be cool, too. Look for 50s style clothing that's a bit on the sexy side, and you can do the make-up pretty easily.

Here's some make-up tutorials:

Pin-up Girl: http://www.youtube.com/watch?v=s2DRyhDLFwo

http://www.youtube.com/watch?v=syjnwqh9VD0

Just have fun with it, and don't worry about what he might think; if he sees how much work you put into it, and sees you enjoy what you're wearing, he'll be impressed.

Gypsy with a short skirt
high boots
lotsa glam
scarves
flowy hair