We have heard the "nothing" answer for a couple of years now. So, I changed my strategy and now I always ask how my child's day went at school/camp/extra curriculars etc. The answer is always "good", "great" etc. Then I ask him to name 3 things that made school etc great. And we pick up from there.
Mostly, he is excited about playing at recess, playing with blocks and legos in the classroom and a book that the teacher read aloud (she selects some good books for their level from scholastic). Sometimes, he mentions that they grew some seeds in a pot or that the teacher bought in a butterfly growing kit etc. I have never heard him be excited about academics in the classroom yet.
