Laying backon the couch, with your girlfriend or best buds, enjoying a large greasy pizza and cold michelob light, while watching a good movie on the tube.
A good time is when you are describing an item, person or place. This isn't the usual meaning of a good time. This is when an item is a good time or that person is a good time. It's specifying a single thing that is a good time just in an odd way.
Same thing goes for "a bad time"
"I like Lucy, she's a good time"
"You should buy this shampoo, it's a good time"
"Lorde's new album is a good time"
"Plaza chips are a good time"
"Uni is a good time"
"Green tea is a good time"