Try to incorporate your feelings and “statement” into something you both enjoy doing together.
A simple card could work. There are a lot of cards out there.
Whatever you do (not sure how old you are) don’t get drunk, eat a bunch of onions, try to kiss her and ask her to marry you! (happened to me.)
Make sure that you pull her aside, make it special. It should be just the two of you in a controlled environment. Be humorous but you must still be sincere or she may not take you seriously.
If you know her little brother you could get him in on the action with you… if he’s a cool kid. You could do the scavenger hunt thing… Just some ideas!