Slightly dreading shrieks of "She's not special, my child was born reading Shakespeare!", but anyway -
DD (21 months) know all her letters (without much input from us) and I'm not sure where to go next with that. As in, she goes up to road signs and says the sounds for all of them. Same when we read books. Same for toys/puzzles/clothes with letters. She clearly enjoys this, and I'm not sure what we can do to help her build on it. Try to show her the link between UC letters (that she knows) and lower case? Show her what happens when you put two letters together? TV shows with letters?
We don't push her (in this or anything else) but she clearly enjoys letters and I don't know what else to offer her to build on her enthusiasm at this age.