Pay attention to the hints she may be dropping & you'll get ideas.
If you don't feel there are enough hints flowing at the moment, tell her you need to run an errand at the mall and invite her. Then do something totally mundane (look for a new drill or some other guy thing). Then kill time just walking around with her - act like you're humoring her enjoyment of window shopping, but pay attention to what she admires in the stores. If you want to surprise her, don't ask questions, such as, "Would you like this?" just listen & file it away. If you want her input, go ahead and ask for more info.
If you really want to spend some bucks & she likes jewelry, tell her your watch needs a new battery (set back the time or something to make it look slow). Then kill a few minutes in the jewelry store to see what she is checking out. Believe me, you'll get ideas.
Let us know what she gets on the big day?