Someday by Charlotte Zolotow was published in 1965 but it's as relevant as ever. It's a very simple book but one any parent with imaginative children will relate to.
A little girl about four of five years old thinks about all the things that she will do some day. Someday her dance teacher will have the class watch her perfect performance. Someday a box of books will arrive when she has nothing else to do.
I came across this book during my cataloging internship. I was immediately struck by the cover showing a joyous girl dancing. On reading the book (which only takes a couple of minutes) I was reminded of my own daughter who must say "someday I will..." or "someday I want..." about fifty times a day.
The illustrations are by Arnold Lobel. He captures the girl's personality and dreams perfectly in his understated way.