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!