Guy I'm seeing invited me to the beach with him and his friends, but I made plans with someone else. How do I let him know without sounding rude?

I want to let him down easy. How's "I'm sorry, I already made plans with a friend to go walking with her. I'd like to join you guys next time. Hope you all have fun." ? Is that a good response? He and I have only been on 5 dates so far. He's a very well-mannered person. Classy in the way he talks.
Guy I'm seeing invited me to the beach with him and his friends, but I made plans with someone else. How do I let him know without sounding rude?
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