The physical journey is the overt meanings of the poem: a man is riding a buggy (because the horse has harness bells) and stops by the woods to watch the snow fall. He has places to go ("miles to go before I sleep") but still makes time to watch the simple beauty of a snowy evening, untarnished by human presence.
The inner journey seems to be one of appreciation of beauty, of the desire to stop working and just be. "The woods are lovely, dark, and deep" can be a metaphor for his desire to have time to explore himself and his own desires and beliefs, but he hasn't time, for he "has promises to keep." The deadlines crowd his mind and take precedence, his responsibilities keep him occupied, taking him away from knowing himself.
The imaginative journey is one of peace and happiness, tranquility. Perhaps, like most of us, he imagines that someday he will achieve time to enjoy beauty for its own sake, without it being tarnished by his need to keep moving, to keep his promises.
The fact that he has "miles to go before [he] sleeps / And miles to go before [he] sleeps," suggests all three journeys at once. He has things to do, deadlines to meet, places to go; he also has much to learn about himself, somewhere along the way (of his life), hopefully before he "sleeps" for the final time, and miles to go before he has a chance to truly rest and enjoy the fruits of his labors in his old age.