Ask which moment she would keep
Choose a year or a period of your marriage and ask what she remembers most fondly. It may be a moment you barely noticed. Describe your own version too; two recollections can make a richer account than trying to agree on a single telling.
Let appreciation become specific
Name something she did and the difference it made to you. Then ask what kind of time together she would enjoy now. Avoid presenting the evening as a reward for household work or assuming a surprise is welcome. A small plan you both want is more useful than an elaborate gesture one person has to organize.