The excerpt you quote should include the beginning lines of that sentence.

There was wide wand'ring for the greediest eye,
To peer about upon variety;
Far round the horizon's crystal air to skim,
And trace the dwindled edgings of its brim;
To picture out the quaint, and curious bending
Of a fresh woodland alley, never ending;
Or by the bowery clefts, and leafy shelves,
Guess were the jaunty streams refresh themselves.
I gazed awhile, and felt as light, and free
As though the fanning wings of Mercury
Had played upon my heels: I was light-hearted,
And many pleasures to my vision started;
So I straightway began to pluck a posey
Of luxuries bright, milky, soft and rosy.

So, the speaker is saying that there was an expansive view which would provide variety for the most demanding nature lover. It was possible from that vantage point to see most of the horizon in its entire 360-degree circle and to make out where the horizon line faded out of sight with the curvature of the earth. Then the speaker describes some of the features of the landscape--woodlands with lanes or alleys running through them, and tree-lined gullies or "clefts" and banks or ledges covered with shrubbery indicating places where there was plentiful water running down the sides of distant mountains.

In the last lines the speaker describes his feelings inspired by the panoramic view. He feels like the Roman god Mercury, who was messenger of the gods and able to fly. (This image may have been inspired by Shakespeare, who has Hamlet describe his dead father  as having "a station like the herald Mercury / New-lighted on a heaven-kissing hill.") The speaker decides to pick a bouquet, or posey, of the wildflowers around him, concluding the stanza.

The poem is obviously inspired by some of those of Wordsworth, such as "I Wandered Lonely as a Cloud." Keats is also obviously extemporizing, trusting in his creative imagination of provide him with images, meters, and rhymes for his couplets as he goes along and not knowing in advance exactly where he is going but having what he described as "negative capability." Part of the pleasure in reading poems like this by Keats is in seeing how his inspiration sustains him and sensing how faithfully he trusts it.