The sentence that best connects the two ideas is the first one: The park benches need a new coat of paint, so we should volunteer to paint them.
This is a complex sentence since it includes two clauses: an independent clause and a dependent clause. They have been joined by means of the conjunction "so" to indicate that benches sould be painted as a result of knowing that they need a new coat of paint. "So" indicates a cause-effect relationship. On the other hand, the other option do not make any sense since the clauses have been joined by "however", an adverb that is used to indicate contradiction, and "although", a subordinating conjunction that connects ideas that contrast. The clauses contained in this sentence do not present ideas that contrast nor express a contradiction; therefore, a conjunction that indicates cause-effect relationship such as "so" must be used.