___it is to go for a picnic on such a fine day! 为什么不能用How fu

因为funny做形容词时的真实意思并不是指中文里的‘好玩’或‘有趣’
funny的意思有以下
1,amusing,making you laugh 好笑的,令人发笑的
例 funny story/joke/film etc
Do you remember any funny stories about work?
2 strange,unusual,strange,or difficult to explain:奇怪的,不寻常的,或者难以解释的
例句 I had a funny feeling something was going to happen.
What's that funny smell?
It's funny how you remember the words of songs,even ones you don't really like.
所以你要表示‘在好天气去野餐是很高兴/有趣的事’最好用 how happy/interesting

？应该是可以的啊，要不那用什么呢？

FUN 和 Funny 的区别，fun 乐趣， funny 有趣，此处 应为 What fun 这里是感叹句的用法