Well, I happen to know you’re already engaged, and she still wants to be proposed to (I know the circumstances where as not everyone on fluther does) , so she may get an idea you may propose if you set the mood a bit much.
I’d say, maybe plan a weekend get away or something, make it a very special weekend, and find the timing, and the location you want to do it at. Try and make it as memorable as possible, I’m sure you’ll both have fun telling future children and grandchildren how you proposed to her.