I’m not sure if you intended the plural present participle "we," or, if you are referring to you and your friends. I have written the paragraph in two different forms for you. Also I recommend clarifying whether or not the trip was cancelled due to the lack of cabin availability. I have removed "What a pity!", it is not as common in current English as "what a shame" (which are both improper usage of the terms), instead adding "Unfortunately"
Plural = We = me + family or similar
Singular = I (self)
Trip Plan (Plural beginning)
We are going to the Great Smokey Mountains with our friends this week. We will be spending 4 days there, unfortunately when I tried to book a cabin they were all unavailable.
Trip Plan (Singular beginning)
I am going to the Great Smokey Mountains with my friends this week. We will be spending 4 days there, unfortunately when I tried to book a cabin they were all unavailable.
Trip Plans
We are will going to the Great Smoky Mountain with our friends this week. We will spend 4 days there. I tried to book a cabin there, but it's was too late, every cabin is booked unavilable. What a pity!