Ward
If you want to go with the mystery, then tell her you have a surprise you don't need to tell her where but do tell you want it to be a surprise. THat way if she picks up on any secrecy she knows it is just romantic.
Is about building the trust rather then being overly romantic, I appreciate the difference and so will she after the fact but before the surprise is sprung the secrecy might alarm her and put her on guard.