I would think small succulents would look nice-keeping things small so you can see the 'fairies'
I agree small succulents
Mini daffodils might be cute ( tete a tete ) mixed in with lots of colorful dainty, airy flowers. I'd go to the garden dept. of your favorite store or a local nursery and gather up what looks pretty together so you will have a good idea of what it all looks like before you plant.
small herbs like creeping thyme and violets work great in fairy gardens