I cannot see what flowers are at my feet, Nor what soft incense hangs upon the boughs,
The speaker remains in the nightingale's nighttime world. (Get it? The night-ingale's home is the night? Keats, you're so clever!)
Without light, the speaker can't see the flowers on the forest floor or the plants that produce that pleasant smell ("soft incense") in the trees. (We don't know if he's talking about the trees themselves or something that grows on them...)
But, in embalmed darkness, guess each sweet Wherewith the seasonable month endows The grass, the thicket, and the fruit-tree wild; White hawthorn, and the pastoral eglantine;
Marco! Polo! The speaker is still groping around in the dark, but he's having fun.
Because he can't see, he has to guess what "sweet" flowers and plants he smells, which depends on what month it is. It's a delicious guessing game.
The darkness is "embalmed," where "balm" is a sweet-smelling substance like a perfume.
He's guessing all kinds of different plants: "Grass!' "Fruit tree!" "Wait, wait, I know this one: white hawthorn! No, it's eglantine!"
Or maybe he smells all of them at once, like a bouquet.
Fast fading violets cover'd up in leaves; And mid-May's eldest child, The coming musk-rose, full of dewy wine, The murmurous haunt of flies on summer eves.
The speaker names more plants that he smells in the darkness. He also begins listing things that he can hear. This section all relates to the experience of being alone in a dark – but not a frightening – forest.
He sees violets, a summer flower, and the musk rose, a flower that blooms in May. The dew of the musk rose is intoxicating, like the wine he spoke of earlier.
He hears the sound of flies on a summer evening.
In short, he seems to experience both spring and summer at the same time, which tells us that we have left the world of strict reality. As Dorothy might say, we're not in Kansas anymore.