The other day, I visited Hokkaido for work, which is the northernmost prefecture in Japan., for work.

Luckily, it wasn't snowy, but all the car lanes and sidewalks were covered with snow.

Took the comma out after "luckily".

If you take out "it wasn't snowy", then you would have the sentence: "Luckily all the car lanes and sidewalks were covered with snow." Which isn't what you thought was lucky.

So to tie "luckily" to "it wasn't snowy" we remove the comma.

I've rarely visited such a cold city, so it was hard for me to choose my clothes and shoeswhat to wear.

Your original sentence is correct, but I replaced it with a more natural phrase.

In fact, I once visited Santa-Claus Village in Finland before, and that which was the coldest place I've ever been.

There was less snow cCompared withto Santa-Claus village there was less snow, but I had to be careful not to slip.

Here its more natural to put the thing (object) at the beginning and then introduce the comparison.

The other day, I visited Hokkaido for work, which is the northernmost prefecture in/of Japan., for work,

No mistake per se but this is the expected word order. Usually you want to explain what something is right after you introduce it. If you were speaking and caught that some people don't know what 北海道 is, you might introduce it later like you did here.

Luckily, it wasn't snowying at the time, but all the car lanes and sidewalks were covered with snow.

In fact, I once visited Santa-Claus Village in Finland before, and that was the coldest place I've ever been.

It's not entirely wrong to omit the "to" at the end but I think it's better to have it here.

There was less snow compared wito the Santa-Claus village, but I had to be careful not to slip.

If I'd had enough free time, then I could have enjoyed its??? beautiful white scenery more!

Not clear which location the pronoun refers to.

The only unique thing I did there was trying S scallops soup on the round flight back.

Is this what you mean? Round trip means both forward and back. For example when you buy the plane tickets and it's both the flight to your destination and back home packaged into one.

Luckily, it wasn't snowy/ snowing a lot, but all the car lanes and sidewalks were covered with snow.

Just another day way of expressing

In fact, I once visited Santa- Claus Village in Finland before, and that was the coldest place I've ever been.

No need for the - in Santa Claus. If you said once visited, i think there's no need to say "before". If you want to say "before", you can remove "once"

There was less snow compared withto Santa- Claus village, but I still had to be careful not to slip.

"Still" gives the sense that even though there was less snow, you had to be careful nonetheless.

The only unique thing I did there was trying Sscallops soup on the round flight.