One approach to this passage is to look at it generally literally, as Jamieson, Fausset, and Brown do. Regarding verse 28, they emphasize the fruitfulness of Canaan:
To an Oriental mind, this phraseology implied the highest flow of prosperity. The copious fall of dew is indispensable to the fruitfulness of lands, which would be otherwise arid and sterile through the violent heat; and it abounds most in hilly regions, such as Canaan, hence called the "fat land"
Palestine was famous for vineyards, and it produced varieties of corn, namely, wheat, barley, oats, and rye.
Regarding verse 29, they point to the success of Jacob's descendants against their earthly enemies, both prior to arriving in Canaan and once there, but also to future spiritual blessings:
[Verse 29 is] fulfilled in the discomfiture of the hostile tribes that opposed the Israelites in the wilderness; and in the pre-eminence and power they attained after their national establishment in the promised land. This blessing was not realized to Jacob, but to his descendants; and the temporal blessings promised were but a shadow of those spiritual ones, which formed the grand distinction of Jacob's posterity.
Calvin likewise calls the promise of Canaan "a mirror and pledge to them of the celestial inheritance." Instead of being focused on temporal blessings, "nothing is further from [Isaac's] mind than to confine the hope of his son to this world."
Church fathers tend to view the entire passage more figuratively. Hippolytus writes:
If one believes that this blessing was accomplished in Jacob, he is mistaken. Nothing of this ever happened to Jacob.
He follows Ambrose sees the smell of Jacob's clothing as accomplished only in Christ: "In fact, the field is the world, and the smell of his clothes are all those who believe in him." Similarly, the promise of the dew of heaven
signify clearly the Word, who came down from heaven like dew. The earth is the flesh that he has assumed from the Virgin.
The grain and wine signify the saints, "who are gathered together like the grain in a barn and are justified by the Spirit as by wine."
Cyril of Alexandria's interpretation is also allegorical: the dew is Christ, and the grain and wine refer to "strength and happiness [...] given to those who are in Christ through him."
These two fathers also see the promises of verse 29 as primarily figurative:
And also the words, "Let peoples serve you, and princes bow down to you" have been accomplished now. Whom else do the faithful peoples serve and the princes of the church worship but Christ, in whose name they also receive their salvation?
Therefore the words of Isaac have been accomplished in the Savior: He has become lord and master of those who are considered to be his brothers by the flesh, in order to be adored by them as their king.