"What did you have for berakfast today?"
你今天早上吃了什么? (what did u ve this morning)
你今天早餐吃了什么?
你今天的早餐是什么?(What was ur breakfast this morning?)
今天 早餐 你吃什麼? (OR) 你今天 早餐 吃什麼?
both need to add “了”, 今天早餐你吃了什么? 你今天早餐吃了什么?
“了”means that it was happened.
if didn't add "了" look like asking ,what do u want to eat for breasfast this morning.
HOPE THAT IT MIGHT HELP U.... :)