1. Find the largest, most remote wilderness area you can. Hike to the middle of it and stay there for a couple nights. Stay up late and look at the Milky Way.
2. Climb up a mountain, sufficiently high to get above the tree line, and watch the sunset in silence. I am partial to the White Mountains of New Hampshire.
3. Take a cruise on the Southern Ocean and watch the phosphorescent wake of your ship at night caused by the stirred up plankton.
4. Go through the Strait of Magellan and the Panama Canal by boat. Contemplate the wonderful creations of nature and of humankind as you do.
5. Go to a foreign country where you don't really know the language. Hop on a bus going anywhere. Get off where something looks interesting. Then find your way back.
6. Pick a battle from some armed conflict. Research and learn all about it. Then, go to that battlefield on a quiet day, sit down and think about what happened there to ordinary people and about the folly and futility of war. Antietam would be a good choice.
7. Spend at least 45 minutes intently looking at some famously awesome work of art in a museum. I recommend Michelangelo's "David" in Florence.
8. Find a place of religious pilgrimage, such as the Basilica of St Francis of Assisi, watch the continuous line of pilgrims coming to his crypt, a line that has not stopped in nearly 800 years. Contemplate the nature of religious devotion in general and of your own religious beliefs.
9. Do something that risks embarrassing yourself in front of an audience, like singing, acting, dancing or even just speaking or lecturing. Prepare for it well and feel great when it is over.
10. Master, practice and take pride in an obscure skill that few other people can do well. For me, that was as simple as learning to shave every day with a straight razor.
11. Someone mentioned genealogy. Trace your ancestry back as far as you can. If you are not descended from nobility and your ancestors came from a country with relatively good records, such as England, that will be to the mid-1500s (so a little less than 500 years) or about 15-16 generations. After you have done that, go to the Pantheon in Rome and see that it was built almost 2000 years ago, in about 126 AD or 60 generations ago. Then go to Egypt and look at the Great Pyramid of Giza, which was already over 2500 years old when the Pantheon was built. So, built by humans about 135 generations past. And finally, go look at the Lascaux cave paintings, which were put there about 17,000 years ago, or about 600 generations of humans past. This should give you a better appreciation of time -- from just how long the human story spans to how very, very short is our individual part of that story. It should inspire you to live and do and see and experience as much as you can while you can.