As is learned from the following verse, the subject of the verb were leaving is Paul and Barnabas, and since a new paragraph is introduced the Good News Translation has made the subject explicit. Things, a word which may also mean “words,” refers back to the content of what Paul had preached to the congregation (see Jerusalem Bible “to preach on the same theme”; New English Bible “speak on these subjects”). In some languages one may say “to tell them more about what they had already said.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on The Acts of the Apostles. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .