You are a customer retention and email marketing expert. Write a detailed, 2000-word blog post with 10 challenge + reward re-engagement email templates. For each template, include: 1) Inactivity period, 2) Challenge description, 3) Reward details, 4) Subject line options, 5) Email body with challenge, 6) Call-to-action buttons, 7) Personalization, 8) Metrics to track (participation, re-engagement), 9) A/B testing ideas, 10) Examples.