Yep you're probably not getting out of this easy...
However in this case I'd say you're not
wrong except through the fact that you're the man, and thus always wrong... You did plan on a nice present, the trip. However you didn't present it at the appropriate time.
So you probably need to acknowledge to her that you were a thoughtless jerk.
...but at the same time show commitment to getting the trip done. Even if it's a homemade trip brochure. What she saw was no commitment to the gift/display of your commitment. You can't fix that. You can make it better. I wouldn't run out and just buy something, it wouldn't mean much because to anyone it would feel like a guilt gift.