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I really like this question. Challenges and adversity can create wonderful opportunities for holistic growth. They make us better problem solvers and hopefully they make us more thoughtful citizens. A lot of people seek the path of least resistance and they miss the beauty and the depth that resistance carves into a person, not to mention the gratitude that comes from overcoming adversity.
I have found one area that really push me and helps me to grow as a person and a leader and it may shock you. It is through serving others or volunteering that I have been stretched the most. Maybe you have studied a servant leadership model and you are familiar with this concept. Serving others sets an example for those following you or even just watching you. You can give back to your community and you may find that you benefit more than the person or people that you are serving. I have found that humbling myself and serving someone else brings about new appreciation for the things that I take for granted. It changes me and I hope it makes a better situation for the person I'm serving. There are many volunteer opportunities out there, meals on wheels, big brothers, Habitat for Humanity, and so on. You can also think outside the box. Tutoring, coaching little league, or mentoring. I love Create the Good for matching you with volunteer opportunities in your neighborhood that match your interests, http://www.createthegood.org/. I hope you find this helpful. May I ask what activities you've pursued on your own that have stretched you?