Would "in my area" be a non-restrictive clause in the sentence, "These are the only two parks I know, in my area"?
Edited and corrected, thanks to @AlanCarmack:
"In my area" is not a clause, it's a prepositional phrase. "In my area" doesn't have a subject-verb combination and does not form a predicate. Instead, it provides additional context to the clause it is in. "In" is the preposition in this case.
On that note, I think that the comma is incorrect here. The sentence should be, "These are the only two parks I know in my area." In speech, there may be a pause before, "in my area", but this should not be signified by a comma in writing.
You could also write it as "These are the only two parks in my area that I know.
In my area is a prepositional phrase. It is not a clause. The whole idea of restrictive and non-restrictive clauses refers only to relative clauses as far as I know, and the insertion of commas is largely stylistic in regard to prepositional phrases. I would never use a comma after know in
These are the only two parks I know in my area
because a comma would totally break up the flow of the sentence.
The true relative clause in this sentence is
[that] I know in my area
and leaving out the relative pronoun that and a comma before it makes the clause restrictive.