I remember it as one of the most impressive memories in all of my life. I remember it as one of the most impressive memories in all of my life.

ImpressiveA Situation During a Trip A Situation During a Trip

About 15 years ago, I traveled around Jeju island by bicycle. About 15 years ago, I traveled around Jeju island by bicycle.

Alternative: "by bike" instead of "by bicycle"

About 15 years ago, I traveled around Jeju iIsland by bicycle. About 15 years ago, I traveled around Jeju Island by bicycle.

Capitalize "island" since it's a proper noun.

Over 3 days, my friends and I rode about more than 200 km. Over 3 days, my friends and I rode more than 200 km.

Your sentence is correct right now, but you can remove "about" if you want to. There is no change to the meaning of the sentence.

Over 3 days, my friends and I rode about more than 200 km. Over 3 days, my friends and I rode more than 200 km.

During the trip, we shared many good memories such aslike riding down a hill path surrounded by flowers, eating delicious local food, and even making friends with other travellers. During the trip, we shared many good memories like riding down a hill path surrounded by flowers, eating delicious local food, and even making friends with other travellers.

During the trip, we had many good memories such as riding a hill path surrounded by flowers, eating delicious local food, and even making friends with other travellers. During the trip, we had many good memories such as riding a hill path surrounded by flowers, eating delicious local food, and even making friends with other travelers.

Alternative: "Befriending other travelers" instead of "making friends with other travelers"

During the trip, we hmad many goode wonderful memories, such as riding a hilly path surrounded by flowers, eating delicious local food and even making friends with other travellers. During the trip, we made wonderful memories, such as riding a hilly path surrounded by flowers, eating delicious local food and even making friends with other travellers.

The most memorable moment iwas when we rode our bicycle durings at midnight. The most memorable moment was when we rode our bicycles at midnight.

"At midnight" is more natural in English.

The most memorable moment iwas when we rode our bicycle duringat midnight. The most memorable moment was when we rode our bicycle at midnight.

We had no choice to ridbike ion the dark path even though it was toovery dangerous. We had no choice to bike on the dark path even though it was very dangerous.

We had no choice but to ride in the dark path even though it was too dangerous. We had no choice but to ride in the dark path even though it was too dangerous.

We had no choice to ride in the dark path, even though it was too dangerous. We had no choice to ride in the dark path, even though it was dangerous.

Aslthough we were tired and nervous, one of my friends said, "Look up at the sky"." Although we were tired and nervous, one of my friends said, "Look up at the sky."

When quoting in English, put a comma after words like "said" "yelled" "cried" etc. And keep all punctuation of the quote inside the quotation marks. For example: The teacher said, "Sowon, you wrote very well today. Good job."

As wWe were feeling tired and nervous, when one of my friends said, "Look at the sky"." We were feeling tired and nervous when one of my friends said, "Look at the sky."

As we wegrew tired and nervous, one of my friends said, "Look at the sky"." As we grew tired and nervous, one of my friends said, "Look at the sky."

We were able to see lots of shooting stars in the dark and beautiful sky. We were able to see lots of shooting stars in the dark and beautiful sky.

Alternative: "a lot of" instead of "lots of"

The stars gave us cheered us up, and we finally arrived to our destination with no acincident. The stars cheered us up, and we finally arrived to our destination with no incident.

"Incident" fits better than "accident" here. For reference: an “incident” is an unexpected event where no one gets seriously hurt/nothing significant happened. An "accident" is an unexpected event where someone does get hurt or something gets damaged.

The stars gave us cheered us up, and we finally arrived our destination with no accidents. The stars cheered us up, and we finally arrived our destination with no accidents.

Alternative: "Safely" instead of "with no accidents"

The stars gave us cheered us up, and we finally arrived our destination with no accidents. The stars cheered us up, and we finally arrived our destination with no accidents.