You have recently been to stay with an old friend for a few days. You hadn't seen each other for a long time.in the letter you should say:
how you felt about the visit
refer to something enjoyable that you did while staying with him/her
invite your friend to visit you
I'm writing to thank you for a really enjoyable visit. It was good to see you after so long, and to be able to spend some time with you, talking about old schoolfriends and teachers.
Now that I'm back home, I keep thinking about how good it was to be somewhere quiet and peaceful. I particularly enjoyed our trip to the wildlife sanctuary and appreciated the opportunity to see so many rare birds and animals. It's made me think that I should try and do more to protect them, so I'm having a look on the internet to see what sort of organisation there are.
Perhaps next time we get together you could come and visit me. There are plenty of places we can go and my brother would really like to see you. Let me know when you might be free to come here and we can fix something up.
Look forward to hearing from you soon.