Early this month, for the first time in ages, I madetook a trip to Sendai, which I had held backput off due to the pandemic.

Thinking that my daughter, a 1st graderyear student in high school, would not come with meus due to school, I initially planned to go on one day trip with my husband at the beginning.

In America, a first year high school student is called a "freshman", but I am not sure if that applies to a Japanese high school

However, she said, "You two should have anstay overnight stay there," so we made a sudden change to our plan, deciding tos and stayed for one night.

My daughter was allowedble to stay over at my gramma living's house nearby.

Also, "grandmother's"

In Sendai, my husband and I spent the day leisurely sightseeing around Sendainear the station.

What amazed us in the hotelmost was the seaweed in a net punet into the hotel bath.

Also, "amazed" = "impressed"

After a little while, my body adapted comfortably to the pale brown bath.

"A while" sounds like a long time, "A little while" is a short time

At the same time, I felt as if I haved melted into soup stock inwith the aroma of seaweed.

Somehow, my skinit also seemed to become moistmoisturize my skin.

While I was on my way home, I thought I would try theat seaweed bath in myat home.

A long-awaited excursion was refreshing and made me newly aware of the goodness ofrealize the value in taking a trip.

Early this month, for the first time in ages, I made a trip to Sendai, which I had held back due to the pandemic.

*which I had been postponing due to the pandemic (this is formal and informal)
* which I had put on hold due to the pandemic (this is more informal)

Thinking my daughter, a 1st grader in high school, would not come with me due to school, I planned to go on one day trip with my husband at the beginning.

Thinking my daughter, a first year high school student, could not come with me because of school.

You can use either could or would not but could not is used when something stops you from doing something and would not sort of means she didn’t want to come. Seems to me you mean that your daughter could not come on the trip because of having to go to school?
If she didn’t want to come because she wanted to do her homework instead, then you can say she would not come.

My daughter was allowed to stay over at my gramma living nearby.

Better to use grandmother instead of gramma

In Sendai, my husband and I spent the day leisurely sightseeing around Sendai station.

Sentence is perfectly ok. You don’t actually need to use leisurely here. Sightseeing itself implies you took your time to explore a place.

What amazed us in the hotel, was the seaweed in a net put, for putting into the bath.

You can also say at the hotel (this would be my preferred way of saying it as a British English speaker)

Feeling thrilled, I bathed in the "seaweed bath."

Your sentence is fine but you can omit feeling thrilled and say
I was thrilled to bathe in a “seaweed bath”

After a while, my body adapted comfortably to the pale brown bath.

Sentence is fine but can you please explain what you mean? Did your skin change colour? Did you feel your muscles relax?

At the same time, I felt as if I have melted into soup stock in the aroma of seaweed.

I felt as though I had melted into a broth/ soup stock, with a distinct seaweed aroma

Somehow, my skin also seemed to become moist.

Please explain what you mean here.
Moist means something that is not dry. This in combination with the bath and the use of “somehow”, which indicates an unexpected thing, doesn’t make sense.

While I was on my way home, I thought I would try the bath in my home.

Do you mean that you will continue to take seaweed baths at home?

Seaweed bBath in Sendai

We capitalize the first letter of important words in titles.

Early this month, for the first time in ages, I made a trip to Sendai, from which I had held backrefrained from going due to the pandemic.

or

Earlier this month, for the first time in ages, I made a trip to Sendai. It was my first trip since before the pandemic.

Thinking that my daughter, a 1st gradeFirst Year in high school, would not be able to come with me due to school, I planned to go on onea day trip with my husband atfrom the beginning.

In the US, we don't use the term, "First Grade in high school."

However, she saidShe said, however, "You two should have an overnight stay there," so we made a sudden change to our plans, deciding to stay for one night.

We refrain from using, "however" and "but," at the beginning of sentences. In the US, we also often refer to "plans" plural instead of "plan" singular.

My daughter was allowed to stay over at my gramma living's house which is nearby.

In Sendai, my husband and I spent the day leisurely sightseeing around Sendai sStation.

What amazed us inat the hotel was seaweed in a net put into thea bath.

At the same time, I felt as if I haved melted into soup stock in thewithin an aroma of seaweed.

While I waswe were on myour way home, I thought I would try the bath in myat home.

Thinking my daughter, a 1st grader in high school, would not come with me due to school, I planned to go on a one day trip with my husband at the beginningfirst.

Although you usually don't use a determiner (words like "a", "this", "the" etc.) before "one", here the "one" is part of the noun phrase "one day trip" so you need an article.

When using one as a number, eg. "one book", "one man", you wouldn't use an article. When its part of a phrase you use an article eg. "the one man show", "a one letter word".

My daughter was allowed to stay over atwith my gramma livingwho lives nearby.

Two ways of writing this:
"...stay at my gramma's house/place/apartment etc. nearby."

"...stay with my gramma who lives nearby."

What amazed us iIn the hotel, wase were amazed at the seaweed in a net put into the bath.

At the same time, I felt as if I haved melted into soup stock in the aroma of seaweed.

AThe long-awaited excursion was refreshing and made me newly aware of the goodness of a trip.

I want to go out with my whole family next time.