About the Hike Last Saturday

The route was along(?) an island in the middle of Taihu Lake.

“Along” would be OK if the island were very long and narrow, or if you went along a feature of the island, such as its coast, crest or ridge.

This island is quite famous; some well-known poets in Chinese history have been there and left many beautiful poems, such as Bai Juyi 白居易 and Fan Chongyan 范冲淹, such as Bai Juyi 白居易 and Fan Chongyan 范冲淹, visited there and left many beautiful poems.

I moved the names closer to “poets” to make it clear that these are the names of two poets, not the titles of two poems.

When I saw the picturesque scenery, those poems I had learned { in my school timedays | in school } just automatically appeared in my mind.

To me, it' was a double delight.

I agree with Liag.

This island is quite famous; some well-known poets in Chinese history have been therevisited and left many beautiful poems, such as Bai Juyi 白居易 and Fan Chongyan 范冲淹.

"have been there" suggests they are still alive. I suggest "visited" or "came here" or "traveled here" in the simple past tense.

I understand "left" to mean they wrote poems and left them for us. However, it looks like not everyone will understand this phrasing, so you might consider alternatives like "were inspired to write" or "wrote" or "produced" instead. You might also add "about it/ the island" for extra clarity. So for example:

"This island is quite famous; some well-known poets in Chinese history, such as Bai Juyi 白居易 and Fan Chongyan 范冲淹, visited and were inspired to write many beautiful poems about it.

When I saw the picturesque scenery, those poems I had learned in my school time just automatically appeared in my mind.

The simple past is fine. if all three verbs in the sentence are in the same tense, the flow is better.

To me, it' was a double delight.

Changing "it's" to "it was" focuses on your experience that Saturday.

If you want to make a general statement using the present tense, the sentence might need more explanation of what the delight is. For example:

"To me, it's a double delight to recall poems about a landscape while hiking through it."

You can have a glimpse of the island from this video: https://www.youtube.com/shorts/iPaDhbiXiSQ

The video the narration was helpful. One correction is a missing word at the end: "I really enjoying" should be "I'm really enjoying."

About the Hike Last Saturday

Is this a hike that you've talked about previously? "About the" makes this sound like it is a follow-up to something which has been talked about before.

This island is quite famous; some well-known poets in Chinese history have been there and left many beautiful poems, such as Bai Juyi 白居易 and Fan Chongyan 范冲淹.

By "left", do you mean that these poets left their poems on the island itself?

When I saw the picturesque scenery, those poems I had learned in myback in school time just automatically appeared in my mind.

"in my school time" isn't wrong, but "back in school" is a more commonly said phrase. In fact, "back" can often be used when talking about periods of time in the past: "back in high school", "back when I was working for that company", etc.