In a Bar of Soap
OK, before you write this one off as one of those hokey unique ways to propose- Stop! It's not so hard to do, and it's not as lame as it sounds! It really is one of the neatest unique ways to propose out there. Many places offer custom soap-making. Find out your girl's favorite scent, and get a little trinket that opens (make sure it's waterproof and hardy!! Give it to her as a gift, and wink, letting her know not to finally open that until you're there. One day, when the soap runs out, she opens the trinket and there's the ring (and hopefully you, down on one knee!)
In a Mollusk
(Note: DO NOT TRY THIS WITH LIVE SEAFOOD!)
If your girlfriend's the mariner or seafood type, try making a Little Mermaid-type proposal! Arrange a setting of some sort of mollusk (clam, oyster, whatever), with one large clam shut tight and elevated above all the rest (or whatever you like- above all, be unique!). When she comes in, let her natural curiosity do the work for you and when she opens that top clam, pop down and propose!
Hidden Diamond Ring
For a real secret agent proposal, consider buying a hidden diamond ring! Some rings have fancy latches or doo-dads on them that slide a smooth band of gold or silver over the diamond. When you propose, give her the ring in its hidden state. Then, as she's admiring it, smoothly and suavely glide your hand over the button and watch her gasp as she sees the gem inside!
With a Moving Picture Frame
Set up one of those digital picture frames, but have each of the pictures inside it be one of the words in your proposal! Bring it out when the two of you together, saying that you want her opinion on the choice of proposals. As she's watching the pictures fade in and out, grab that ring from your pocket and get down on one knee! Or, for even more unique ways to propose, have one of the photos in the frame be of you holding a sign asking her to marry you!
In Her Pillowcase
Before you guys go to bed at night (or before a nap, or whatever situation this works best for you in!), slip the ring in her pillowcase. When she puts her head down, she'll feel it against her head and wonder what it is. When she pulls it out and realizes what it is, pop the question!
At a Raffle
This one requires a bit of preparation, but it's totally worth the wait! Arrange with some local organization that holds raffles to raffle you off. When the two of you get to the raffle, note her ticket number and tell the organizers. At a pre-determined point, raffle a very special marriage to you off to the lucky winner (her, if you did this right) and pop out the ring as she gets to the stage!
In a Snow Cavern
You'll have to be somewhere with fresh and heavy snowfall for this! The day it snows heavily, dig a hole from the front door out somewhere (make sure it's covered!). Line the inside of the snow trail with something to indicate where she has to go, and when you hear her approach light some candles. The candles will reflect off the snow, giving it a wonderful and romantic feel!
In a Hollowed-Out Book
Some unique ways to propose take some investigative work. Figure out what book she's reading now or what her favorite book is. Buy an exact copy of the edition she has, and hollow out the inside of it. Put the ring inside the hollow, and close the book and let her find it (While you're there, of course). Think of creative framing for it, too. You could make the book's title her name and a page before the hollow "Next Chapter!"