My DD wasn't an early writer, but she was an early reader. And from the time she was very little she LOVED numbers and letter as you described. One of her favorite gifts was those foam letters and numbers that you can play with in the tub. We had 2 sets so we could make more words. It started with us telling her to "find the letter ..." and quickly progressed to her saying things like, "Spell a fruit!" Then she'd try to figure out what it was. Before too long she was spelling words for us.
Warning - it does make for an extra-long tub-time
