If you're that into to her don't let the plans of a themed event ruin it. Sure Haloween would have been a cool time to do it but you don't need a special occassion to do something nice for her.
First, stop sulking go out and have a good time on haloween. Plan something else you can take her up to that hill any night and reveal the marriage date. I'm sure you'll do fine. Don't put yourself down over it, she probally just wants to spend time with her family.
Also Caff's plan is pretty cool, difficult but if some guy did that to me I would be all over him and I'm straight. ;)