Our speaker spends the better half of this poem imagining what his lover could be doing, but is not. His reverie creates a string of images that we get the pleasure of following throughout the poem. While most of them do not go into great depth—he brushes from one hypothetical situation to another—in the final stanza, when we're in the real moment, Wilbur leaves us with a vivid picnic picture so bracing, sweet, and salty that you can almost taste it.
Stanza 1: Right from the get-go Wilbur releases the hounds of his imagination. You'll be noticing the word "could" a lot in this poem. The speaker imagines that his lover could be sitting in a library reading some old book, or riding the elevator through a department store.
Stanza 2: The speaker's still in la-la land, imagining what she could be doing: planting in a garden, having lunch with a friend who's blabbing about her love life.
Lines 9–11: This guy has no limits to his imagination. He jumps from the luncheon scene to picturing his lover training a dog, to picturing her sitting in a lecture hall.