The appropiate response is well, how would you rate your day Mrs Brown? you liked the art at the museum didn't you?
So, This response allows Mrs. Brown to share her own thoughts about her day, and it also politely suggests that the son should ask his mother directly about her day.
As for the comment about Mrs. Brown making a mess of her candy bar on the bus, it's best to avoid bringing this up unless Mrs. Brown herself mentions it. It's important to be respectful of people's privacy, and it's not always appropriate to bring up embarrassing or negative topics in front of others.