Here is a nice little steganographic riddle: (you could write on the box for instance)
Digits and binary are an example of me
A tad backward, but all part of a plan
Tada! Unscramble to discover with glee
A present I hide, from your old man
In the box you could then have the numbers 4 1 20 1 - which is data and also the number of the letters position in the alphabet for the word DATA.
Explanation:
The first letters say DATA (if thats a bi obvious you could have last letters, bold letters, italics etc.)
Digits and binary are an example of me
Digits and binary are examples of 'data'
A tad backward, but all part of a plan
'A tad' backward spells 'data', which is part of a mobile phone plan
Tada! Unscramble to discover with glee
'Tada' is an anagram (unscramble) of 'data'
A present I hide, from your old man
This is a gift from her father - the OP (using the expression 'old man' as a term for father, not because I am saying the OP is old :P )