simple = easily done or understood; not complex or complicated

• There are three simple steps to follow.
• The answer is really quite simple.
• simple life, simple design, simple solution, etc.
  

simplistic = treating complex issues and problems as if they were much simpler than they really are;
not complete or thorough enough

• A simplistic analysis of the issue.
• They have a simplistic point of view about the war.
• A simplistic approach to a complicated problem.