I enjyoyed a sauna with my friends today.

I don't believe "I enjoyed a sauna with friends today." is necessarily wrong, I just think that it feels incomplete without a qualifier for "friends". Alternate suggestion:

"I enjoyed a sauna with some friends today."

One of the friends haves a sauna tent, and we tried it near a river.

A few alternate suggestions:
"One of the friends has a sauna tent which we tried near a river."
"One friend had a sauna tent and we tried it near a river."

Outside sauna iss are becoming popular in Japan, but someone still think it i's difficult.

When referring to a general concept instead of a particular thing, e.g. saunas in general instead of 1 specific sauna, you usually use the plural version of the word.

Also, almost everyone uses the contraction of "it" and "is" (which is it's) in both written and spoken English (at least, in American English).

We were very relaxinged.

I enjyoyed a sauna with friends today.

Spelling.

One of themy friends haves a sauna tent, and we tried it near a river.

"the friends" feels a bit impersonal. It's how you would describe someone else's friends, but not your own. So it's best to refer to your own friends as "my friends".

Outsidedoors sauna iss are becoming popular in Japan, but someon people still think it i's difficult.

I'm not quite sure what you mean by it being "difficult". Usually "difficult" is used to indicate that a situation is challenging, but this sentence isn't presenting such a situation.

Today, the weather iwas pretty good and warm.

To a lot of people "good weather" already implies that it is warm and not raining.

We were very relaxinged.

Here you should use an adjective, thus "relaxed" is the best choice. You could also say "it was very relaxing".