In the first stanza of "Ode on a Grecian Urn" by John Keats, the speaker refers to the urn as an "unravish'd bride of quietness," a "foster-child of silence and of slow time," and a "Sylvan historian." He speaks to the urn using apostrophe and is clearly comparing the urn, decorated with specific scenes, to something else, which is exactly what a metaphor does.
The next few stanzas are more like descriptions, and the scene depicted in the second and third stanzas is a young musician who is serenading his love under a glorious tree. The young man is leaning forward to kiss the beautiful young woman as the sound of the pipe (flute) surrounds them.
Heard melodies are sweet, but those unheardAre sweeter; therefore, ye soft pipes, play on;Not to the sensual ear, but, more endear'd,Pipe to the spirit ditties of no tone:Fair youth, beneath the trees, thou canst not leaveThy song, nor ever can those trees be bare;Bold Lover, never, never canst thou kiss,Though winning near the goal yet, do not grieve;She cannot fade, though thou hast not thy bliss,For ever wilt thou love, and she be fair!
This is a stanza of wistfulness and consolation. The speaker encourages the pipe to keep playing its song of love, and then he tries to console the young man who is destined never to win either his kiss or his love. Though the young man's love will never be returned, he can take comfort in the fact that his lover's beauty, like the leaves on the trees, will never fade or fail; in the came way, his own love for her will never die.
Unlike the first stanza, however, the speaker addresses the people in the scene decorating the urn, not the urn itself. When a poet speaks to someone who is not real or present as if he were, it is called apostrophe, and there are two examples of this in stanza two of the poem (three if you want to count his statement to the pipe to keep playing). First, the speaker assures the "Fair youth, beneath the trees," that his (the young man's) his song will be able to live forever and the trees will never be bare. Second, he reassures the "Bold lover" that, though he will never be able to kiss the woman he loves (have a fulfilling life with her), she will always be beautiful and his love for her will last forever.
So, while there are no specific metaphors which describe the actual urn in this stanza (that is, no specific comparison between the urn and something else), the speaker does address the figures on the urn by his use of apostrophe. These references are similar to what he does in the first stanza, but they are not metaphors describing the urn.