as a general rule I would tell them of Jesus and his love, but if they specifically ask me about the sabbath, I would answer their question, but in the context of Jesus being the Lord of the sabbath, Jesus is our saviour not the sabbath!
we can teach the fundermentals because the way are presented in the 28 fundermentals focuses on God and Jesus throughout. we can present doctrine without being dry, if we study in advance how best to present.
People need to feel that we are friends to them first and that is what God has called us to be. '' As we do to our fellowmen, we have done it unto Christ.''
Just think about it. When you are friends with people, you feel free to discuss easily and thus we can talk about God's love and saving grace. As our friendships develop, so should our discussion and we can go into talking about our fundamental beliefs.
That saying, there are cases, where you sometimes need to delve right into the meat of the situation, but generally, my forst position holds.
Jesus met people where they are so for Nicodemas, he opened the discussion with a theological point, you must be born again, he scratched where Nicodemas needed to itch. and built a relationship from there.