How exciting!!! Congrats to you and your GF!
My DH proposed to me in front of Cinderella castle the day we got there. I knew about the trip and was hoping for an engagement but once we got there, those thoughts flew out the window because I was too excited to be in WDW again.
He told me to wait (by the entrance) and went to get a cast member (telling me he wanted them to take a picture of us in front of the castle). So they come back over and part of the entrance was roped off so we went to the area that people couldn't go (CM said it was OK, know what was up) and DH got down on one knee. I thought he was just being cute at first until he pulled out the ring box and said "I have a question for you."
It was so cool and I'm so happy that we have pictures of the whole thing. People that were walking through saw what was going on and stopped to congratulate us and hug me. It was so special!
You will have a story to tell forever no matter when/where you do it.