Acts 15 20: The commandment here to abstain from things strangled and from blood was not just decided upon in order to avoid clashes between the Gentile believers and the Jewish believers. This is a part of God's law that is still in affect today. Eating animal blood isn't good for you. Stay away from meat where the carcass wasn't properly drained.
Acts 16 17: A lot of the naive preachers we have today would have been overjoyed at what this demon possessed girl was saying. However, Paul knew that her words would only be a hindrance to the cause of the Gospel because they were said in mockery.