Paragraph 1: 

Dear Mike,
I was thrilled to get your invitation to come camping with you and your family. I'm always looking for opportunities to go outdoors and spend time in nature, and I know that I can count on you to be a gracious host. You really know how to plan a trip that's fun for everyone involved!
I haven't given any thought to my summer plans yet, but I have no doubt that they'll include some time with you all. As soon as I've got the details nailed down, I'll let you know what my dates are.
I'll look forward to hearing from you soon.
Sincerely.

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Dear Mike, I am so excited to hear that you want me to come camping with your family! Where are you going? And when will it be? It would be great to know what dates and times work best for you. I'm really looking forward to this. I've been wanting to go camping again for a while, so I'm happy that it's finally happening. You know I like doing just about anything outdoors, and I love spending time with you and your family. Camping is the perfect way for us all to be together in the great outdoors. There's so much we can do! We can hike through the woods, roast marshmallows over a fire, play games, there are so many possibilities! Let me know as soon as you can where and when this camping trip will take place so that I can make sure my schedule is free for it. 

Your friend, Paul.