The girl I like said it was a great day. How should I reply to her?

How to reply should not be based on your own ideas. She said that the weather is so good today, maybe it's just an ordinary sigh, because that's what she thinks, so she said it. It may also be a hint to you that the weather is so good, do you want to go out for a spring outing together? So you can reply "Yes, I haven't seen such good weather for a long time" (I don't recommend it personally, it feels a bit stiff), or you can reply "I heard that flowers are blooming somewhere. Shall we go shopping together this afternoon? " Choose according to your specific situation, come on.