Thank you for everything! Your father showed my friend and me all of Koln, Germany.
He explained everything and gave us lots of gifts!!!! He also treated us all to a meal.　Wow!
He is the best German man I have ever met. He is a gentleman and really smart.
I really appreciate him so I want to send him a letter and little gift from Japan.
Could you give me his address, please?

Your daughter also took care of us during this travel.
Her room is so elegant and bed is really confortable.

Thank you very much.

With all of my love

まゆみさんへ、

こんにちは。これは僕の訂正版です。Sorry if it is too late :(  The notes are just my suggestions, not necessary.

Thank you for everything! Your father took my friend and me to through all of Koln, Germany. 

He explained everything and gave us lots of gift!!!!And also he treated us all to a meal.　Wow! 

He is the best German men man I've (or I have) ever met. He is a gentleman and really ("really" is correct, but I would prefer to use "very" in this sentence) smart.

I am really appreciated appreciative to of him so I want ("want" is correct, but I would prefer to use "would like" in this sentence) to send him a letter and a little gift from Japan.

Could you give me his address, please? (Here you could also use "Could you please give me his address?")

Your daughter also were taking took care of us during this our travel (I think "travel" may be correct, but "trip" is better)

(plural)Her rooms are so elegant and her beds is are really confortable. (SP)

OR

(singular) Her room are is so elegant and the/her bed is really confortable. (SP)

Thank you very much.

wWith all of my love,