If you're giving it to her in person, you could do something like a "scavenger hunt", but instead of having her look for paper clips or whatever, you can write out clues to the location(s) of the cash. You can have a single $50 bill, or use ones, fives, tens, twenties, or a combination and have lots of different hiding places for her to find them. Or if you're sending it to her, you could enlist her mother's help to hide the money in their house somewhere and you could still do the scavenger hunt to make her find it.
Or, as a variation of that theme, you could send her a puzzle or something she had to figure out (a word scramble, find-a-word, or crossword or something like that) that would direct her to the location of her present (it could be something generic like "kitchen" or "bathroom" -- something that every home has).
You could send her a book, with bills put between random pages. Or put the cash in a napkin ring.