I think it's fabulous that you want to share this wonderful event with her, and I agree that your presence is gift enough. However, I also understand your desire to bring a gift.
I suggest bringing a hand-made gift. Here are some ideas that are free or really cheap:
- a set of word puzzles to entertain them en route to the honeymoon (Mad Libs, word search, scrambles, etc)
- homemade honey mead (if you have access to cheap honey and empty bottles)
- cake mix/cookie mix in a jar (the just add wet ingredients type)
- a little cookbook of your favorite recipes
- a holiday decoration that would be unique to them
- some herb or potpourri sachets from your garden
- a storybook about how they met and fell in love