Of course you’ve already talked about getting married “some day” or “this year” or “in the summer”, so no surprises about that… she knows you’re going to propose but she has no idea when it’s going to be and how it’s going to be.
Your mission is to plan a perfect day without her finding out about it or guessing what’s going on too early during the day, and there are two perfect timings to pull this off:
She is already expecting a romantic celebration so if you take her to an ultra fine restaurant she will not think it’s unusual. Make it an official, non-surprise, date night. You don’t have to disclose the location but do mention that you’re taking her out to a birthday dinner at an upscale restaurant, with a dress code. She will get all dolled up for the night and will look her best at the big moment and the after pictures… and she will thank you for that big time!
Another important step in the plan, to guarantee the perfect surprise: give her the Birthday gift & card at home, before heading to the restaurant.
This way she’s already getting a gift from you so she will not expect or suspect anything later, and will just assume the two of you are going to enjoy a nice romantic Birthday dinner. She is going to be so surprised….
Alternatively, instead of a birthday dinner, you can safely pull a lunch cruise or a picnic in the park, when she’s already expecting a birthday hangout, after receiving her gift at home.
On just a regular or a long holiday weekend getaway, planned weeks ahead, you have so many opportunities for some unusual activities that will not be suspicious, cuz hey, you’re on vacation!
If it’s a road trip, a nice long hike with that perfect viewpoint will be a very normal and expected plan, that she will not suspect, as oppose to “hey baby, we should start hiking, how about Tuesday?”, if you normally don’t.
If you’re travelling to a new area, it will be very natural to include that “very famous Japanese Garden” visit in your sightseeing plans, as oppose to “honey, remember that Japanese Garden that I told you about, I want to check it out next weekend”, if you’re normally not into this kind of activity.