Couple, as a noun, indicates two of something and should not be used as a synonym for a few or some.
Traditional and formal preference treats couple as a noun, thus requiring the preposition of to modify another noun: A couple of bandits rode into town. In the past century, though, casual use has treated couple as an adjective, allowing it to modify a noun without the preposition in-between: A couple bandits rode into town. This noun-to-adjective shift has become increasingly common and will likely dominate within our lifetimes, but for now consider couple of superior to couple in situations that call for more formal writing. In more casual writing, weigh the rhythm and sound of couple against couple of to decide which better suits the prose and its intended audience.