Metaproposal

He asked, she accepted.    …would be far too simple!

So instead:

We agreed about what, got stumped by how.
    (How to devise a collaborative surprise?)

She had an idea: why not propose a proposal?
But before she had a chance to take a chance, he took his chances!
    Naturally, she needed more time.
    He good-naturedly granted it.

Plans spawned more plans (and people, places, props, puzzles):
    A book was made. Clues were laid. Friends came to aid.

Finally, at the end of a puzzling day:
    She asked him to ask.
    He accepted (on the meta level).

Then, and only then:
    He asked, and she accepted.

(Which was pretty simple, after all!)