What is the thing or activity that makes life the most enjoyable, the most meaningful? Of course we could make a list but that would just sound like any other motivational self help book.
I think that the most enjoyable and sustainable thing is the idea of working towards improvement. This can entail building for the future, planning for a better future, learning something new, developing friendships, working hard to become even better at things you care about. Many other good things get dragged along for the ride when doing this, but this forward thinking idea of betterment is the best underpinning. If one does not have a good future looking view, life still goes on, but something huge is missing and things feel a bit trivial.