Lit. the things of peace. So the next clause, things of edification. See on build you up, Act 20:32. Edification is upbuilding.
One another (τῆς εἰς ἀλλήλους)
The Greek phrase has a defining force which is lost in the translations. Lit., things of edification, that, namely, which is with reference to one another. The definite article thus points Paul's reference to individuals rather than to the Church as a whole.