First of all, to make sure we are on the same page, I define metaphysical conceit as an attitude that is expressed through a rhetorical argument, like a thesis while using striking or unusual examples. This approach allows the poet the opportunity to be witty proposing something absurd or indecent. The play about the poem makes the reader drop their guard to see the issue from a fresh perspective.
In "To His Coy Mistress" he warms his audience with the hyperboles of time and space noting her ability to play hard to get. He compares the length of time he's waited to forever:
"Love you ten years before the Flood
And you should, if you please, refuse
til the conversion of the Jews.
He notes that he is willing to allow his love to grow for a long time, but after a while, time can become too long: he spends a stanza on how time can turn into death if they aren't careful. This is how he keeps the poem light and witty.
His ulitimate proposal or thesis comes in lines 33-46. He here uses light in several ways. First, he notes her "youthful hue" and the brilliance of her skin again as he says "at every pore with instant fires." This use of light paints her as a glowing piece of perfection. Often, light is used to reveal truth, here he notes how light reveals her beauty. Finally, after he makes great advances and suggestions to get together and "sport" while there is yet time, he notes the passage of time through the image of the sun going and going and going:
Thus, though we cannot make our sun
Stand still, yet we will make him run.
These are two interpretations of the metaphysical conceit approach to light in "To His Coy Mistress". His ultimate proposal is to the expression of love or sex and the absurd suggests of time and space and death build up to his opportunity to present her as light and their relationship as light by the time he makes his great proposal.