I like it most because it is the best . The couple are spending its honeymoon in Hawaii. The couple is spending their honeymoon in Hawaii. The couple are spending their honeymoon in Hawaii. The couple is spending its honeymoon in Hawaii.
Couple exists as a singular and a plural noun.
The pronouns should be used accordingly. A couple days went by. A couple of days went by. There were less people there than I thought. There were fewer people there than I thought.