Let's look at your sentence: "Ah, that’s a great idea for a gift! Have a nice day!"
It's very well written! Here's why:
- "Ah," is an expression showing you like the idea. It's correct with the comma after it.
- "that’s a great idea for a gift!" is a full sentence with proper punctuation. It means you think the idea is very good.
- "Have a nice day!" is a polite way to say goodbye and it is also written correctly.
Overall, your sentence is grammatically correct and clear. Great job!