Plan a scavenger hunt with clues leading to different locations, each with a small gift or a note. The final destination can be where you ask her out in person.
If you know her favorite coffee shop or restaurant, arrange to have a message written on her coffee cup or dessert, asking her out.
Create a playlist of songs that spell out your invitation when played in a specific order. Share the playlist with her and suggest you listen to it together.
Slip a note with your invitation into a book she's reading or leave it hidden among the shelves for her to find.
If you're artistically inclined, paint or draw a picture that includes your invitation and present it as a gift.
Create a mock movie poster or a short video trailer with you as the star and your date idea as the feature film. Share it with her as a fun and unique invitation.
Plan a night of star-gazing and print out a star map with a constellation that spells out your invitation. Show it to her as you both enjoy the night sky.