Today, my family and my parentsI travelled to Danyang, whereich is a nicebeautiful place in Korea. Today, my family and I travelled to Danyang, which is a beautiful place in Korea.

Your parents are your family Its better to use another describing word other than 'nice'

Today, my family and my parentsI traveled to Danyang whereich is a really nice place in Korea. Today, my family and I traveled to Danyang which is a really nice place in Korea.

About 20 years ago, we visited here and I had, so I've got a lot of good memories. About 20 years ago, we visited here, so I've got a lot of good memories.

I had a lot of good memories means you had the memories, but you don't have or remember them anymore. When in fact you do

About 20 years ago, we visited here andWe had visited here about 20 years ago as well, therefore I hadve a lot of good memories. We had visited here about 20 years ago as well, therefore I have a lot of good memories.

When I came here againreturned here, I cfould find places whereich I had previously visited before. When I returned here, I found places which I had previously visited.

Sounds better, is more grammatically correct

When I came here again, I could findame across places whereich I had visited before. When I came here again, I came across places which I had visited before.

We hadtried a variety of tasty foods and we also visited nice tourist attractions. We tried a variety of tasty foods and we also visited nice tourist attractions.

I hope all of my family members gehave now got good memories here. I hope all of my family members have now got good memories here.

They are not creating the memories now, you want them to now have the good memories

I hope all of my family members getmade good/great memories here. I hope all of my family members made good/great memories here.