How I spent the weekend.

I spent the weekend well. I relaxed on the coast of a big lake. Usually I go fishing there, but that weekend I swam for a few hours. This lake is very old and it is 10 miles to the nearest town. I think, this lake is one of the biggest and the oldest lake in our area. If you found an error, correct me please.

How I spent the weekend.

I spent the weekend well. I relaxed on the shore of a big lake. Usually I go fishing there, but that weekend I swam for a few hours. This lake is very old and it is 10 miles to the nearest town. I think that this lake one of the biggest and the oldest lakes in our area. If you find an error, correct me please.

I changed coast to shore.  A continent has a coast with an ocean.  A lake would not have a coast, rather a shore.

Although grammatical, the comma after "I think" is awkward.  Here is where the word "that" should be included.

Although "found" is not ungrammatical, the request is for an action that I am doing at the moment and therefore, "find" sounds more natural.