Complex sentence: Cook spent only one day in the area and did not land on the mainland but the next day on 16 May, Cook named Cape Byron and Mount Warning (or Wollumbin), after seeing the mountain from the sea while sailing past.
Split into simpler sentences: Cook spent only one day in the area, and did not land. On the next day Cook named Cape Byron and Mount Warning (or Wollumbin), after seeing the mountain from the sea while sailing past.