I traveled to Japan with my friends (last weekends / during the last few weekends).

"weekends" plural would be over multiple weeks, which sounds a little weird just left as is so the longer version sounds more natural.

We lived in a hotel which is located next to the sea in Sharahama.

Japanese ispeople are very polite and considerate.

Just "japanese" on its own sounds like you're talking about the language, not the people

There hasYou can see the blue sky, blue sea and fresh air, which make me feel relaxed.

We tried seafood, Ssushi, Rramen and Ccurry Ppork Ccutlet Rrice.

I lovediked the seafood best because they are so fresh and fat.

We don't really use "loved" and "best" together

We visited to Sandanbeki and Senjojiki.

I feelt peaceful and lonely when I saw the starry sky and endless sea at night.

I traveled to Japan with my friends last weekends.

We livstayed inat a hotel which located next to the sea in Sharahama.

The Japanese ispeople are very polite and considerate.

There hais blue sky, blue sea and fresh air, which make me feel relaxed.

We tried seafood, Ssushi, Rrame and Ccurry Ppork Ccutlet Rrice.

You can say “katsudon” too

I loved seafood the best because they areit is so fresh and fatty.

We visited to Sandanbeki and Senjojiki.

I feelt peace and lonely when I saw the starry sky and endless sea at night.

Tenses match

I traveled to Japan with my friends last weekends.

"weekend" is singular here. Everything else in the sentence is perfect.

We livstayed in a hotel which was located next to the sea in Sharahama.

"lived" is for long-term accommodation. "staying" is used to describe short-term accommodation, like trips.
"which was located" < The past tense of "to be" is best, but "is" can also be used here.

Japanese isare very polite and considerate.

plural form of "to be" is the right choice for a group. It would be even better to say, "Japanese people are..." or "Japanese culture is..."

Japan's environment is very clean and neat.

Perfect!

There hwas blue sky, blue sea, and fresh air, which makde me feel relaxed.

The verb "to be" is correct here for "There was blue sky". Putting it in past tense "was" helps it sound like a recollection of the trip. Same with "make" to "made".

We tried seafood, Sushi, Ramen, and Curry Pork Cutlet Rice.

This is an example of an Oxford Comma. It's generally the accepted rule to put a comma on the second to last item in a list, before the final item starting with "and".

I loved the seafood the best because they areit was so fresh and fat.

Past tense makes this line more consistent with the rest of the story. "Fat" is an awesome but uncommon way to describe seafood. Words like tender, juicy, or buttery are more common.

We visited to Sandanbeki and Senjojiki.

"We visited" or "We went to"

I traveled to Japan with my friends last weekends.

We livstayed in a hotel which was located next to the sea in Sharahama.

The Japanese ispeople are very polite and considerate.

There hais blue sky, blue sea and fresh air, which make me feel relaxed.

I loved the seafood bethe most because they areit was so fresh and fmeaty.

I'm not 100% sure what was meant by "fat" but I'm assuming it meant there was a lot of flesh on the seafood so I used the word meaty.

We visited to Sandanbeki and Senjojiki.

The scenery iwas very beautiful and spectacular.

Try to stay in the past tense throughout.

I feelt peace and lonelyiness when I saw the starry sky and endless sea at night.

I traveled to Japan with my friends last weekends.

We livstayed in a hotel which was located next to the sea in Shairahama.

The verb "stay" is commonly used when talking about where you're lodging temporarily, such as a hotel. "Lived" is generally used for a more permanent or longer-term residence. "Stayed" sounds more natural for a short trip.
"was" to indicate that it's something in the past.
I think you're referring to Shirahama beach.

The Japanese isare very polite and considerate.

When referring to people, the plural form "are" should be used, not "is," because "Japanese" refers to people, not an object or singular entity. We also typically use "the" before nationalities to refer to the people of a country.

There hwas blue sky, blue sea, and fresh air, which makde me feel relaxed.

When describing a past experience, "there was" is the appropriate phrase.
A comma is added before "and" for proper punctuation(following the Oxford comma rule).
The correct verb form here is "relaxed," which is the past participle of "relax." The sentence needs to be in past tense, so "make" is changed to "made," and "relax" is changed to "relaxed," which is the correct adjective form.

We tried seafood, Ssushi, Rramen, and Ccurry Ppork Ccutlet Rrice.

"Rame" is a typo.
I also made the capitalization consistent by using lowercase for all the items

I loved the seafood bethe most because they areit was so fresh and fatty.

"Loved seafood best" is understandable, but "loved the seafood the most" sounds more natural in English. Also, "fat" should be changed to "fatty" because we use "fatty" as an adjective to describe the texture or quality of food. "They" is replaced with "it" since "seafood" is an uncountable noun (referring to the food in general), so the singular pronoun "it" is more appropriate.

We visited to Sandanbeki and Senjojiki.

The verb "visit" does not require the preposition "to" when referring to places.

The scenery iwas very beautiful and spectacular.

Since the rest of your narrative is in the past tense

I feelt peaceful and lonely when I saw the starry sky and endless sea at night.

The sentence shifts to the past tense, so "feel" should be changed to "felt." Also, "peace" isn't typically used as an adjective, so "peaceful" is a better choice to describe the feeling.