In Sonnet 116, Shakespeare employs synecdoche in lines 1-2: "Let me not to the marriage of true minds / Admit impediments." Synecdoche is the use of a part of something to stand in for the whole thing. It isn't just people's minds that get married, it is the whole two individuals that wed. Therefore, "minds" stands in for the whole person. It's a somewhat ironic, or unexpected, choice because we typically think of romantic relationships as a joining of two hearts, if we use any part of the body to describe it. Irony occurs when reality differs from what we'd expect, and since we tend to associate the heart, not the mind, with love, such a choice is ironic.
Further, Shakespeare personifies "Love" as well as "Time," giving them human attributes and raising this story of love almost to the mythic since he ascribes intention and consciousness to intangible entities.
Shakespeare employs synecdoche again in lines 9-10: "Love's not Time's fool, though rosy lips and cheeks / Within his bending sickle's compass come." The sickle is a symbol of mortality (since we only have so much time before we are, figuratively, cut down by it), though time does not only claim our "rosy lips and cheeks"; again, it claims our whole selves. But this example of synecdoche allows Shakespeare to employ a visual image as well.