For puzzles you can use to hide or lock a clue try:
A Wine Bottle puzzle and put a clue in an empty bottle - maybe an object, maybe paper. These are easy for people with experience, but can be really hard if she isn't familiar with disentanglements.
Or a puzzle designed to hold money and put a written clue in it instead. Again, some are easy, some are hard.
You could potentially use the Hanayama Padlock puzzle somehow - does anyone know if having something 'locked' by it prevents you from solving it?
Or get a puzzle box - there are many - some are easy, some are hard - but you'd have to solve it yourself first to put something inside
If you just want a puzzle for her to solve, search 'brainteaser' on Amazon or 'Hanayama puzzles' for generic brain teasers.
Something special might be nice - math related!
Harder boxes
http://www.sloyd.fi/cent-p-1315.html
http://www.sloyd.fi/centrale-p-1840.html