What an important question, and one that we as humans and as Jews return to each generation. Unfortunately, there's no truly pithy answer. There is an interesting discussion in the Talmud, Makkot 23b-24a, where the Talmud envisions various prophets as trying to boil down the Torah and its commandments into ever-smaller units. My favorite in that list, the one that I put on my father obm's tombstone, is Micah 6:8's call for doing justice, charity, and walking humbly with God. The Talmud itself goes further, ending with Amos' note that the righteous shall live by his faith. That faith, though, is an active one, one in which the believer seeks to act in the ways God wants, not just to believe.
I have actually spent awhile discussing these kinds of issues, most recently in a series of posts at blog.webyeshiva.org, called the Mission of Orthodoxy project. There are 21 so far, with about six to come after Passover, but the overall thrust is that all of Judaism is focused on guiding human beings (Jew or non-Jew-- Judaism has a very clear picture of what valuable non-Jewish life would look like as well) on how to develop themselves and their societies in Godly ways.
So that, in very brief sum, the essence of a good life is trying to get ever-closer to God, a process that involves shaping ourselves in ever more Godly ways. To know what those are, a lot of study is required, since it is not always intuitive what being Godly is like. While many times what seems intuitive is in fact what God wants, sometimes things we think of as problematic are in fact the more Godly way to act (and other times, the wrongs we dismiss lightly are in fact much more serious).
The Talmud's other candidate for a single-commandment encapsulation of the Torah is "Seek me and live," which it rejects only because that might mean that unless we fulfill all the commandments, we are not really seeking God (which is, in some sense, true, but defeats the goal of an encapsulation). So, bottom line, brief description: a "good life" is one lived in the constant search for what God wants, by studying the Scriptures God gave us, with the authoritative interpretations passed down over the generations, all of those combining to make us more Godly, which includes, at least, more kind, more concerned with others' welfare, and with trying to make a world of kindness and justice mixed together in the best possible way to usher in a time when all recognize God's Rule and try to shape their lives accordingly.
To answer your question on what constitutes the “good life” for Jews, I point to a single word in the Torah and a single book in the Bible. And the latter qualifies the former. The culmination of the priestly blessing is “shalom” (Numbers 6:26). “Shalom” is not simply peace in the sense of the absence of war. “Shalom” is “wholeness” from the Hebrew root “shalem.” The ultimate blessing is feeling whole, at peace. To achieve that desired end, the Book of Ecclesiastes is instructive. In short, the narrative tells of a person of means (who tradition identifies as King Solomon) who tries every conceivable activity to gain satisfaction: wine, debauchery, music, philosophy. Yet everything he tries falls short. He even contemplates taking his own life yet realizes that will make no difference in the scheme of things. Rather than cynically conclude that life is meaningless and happiness is unattainable, the author comes to the surprising conclusion that in the end, the only thing that is worth pursuing is the Torah! In the penultimate verse of the Book (12:13), he says: “…all things having been heard: revere God and keep His commandments for this is the whole man.” If we are, like Ecclesiastes, searching for purposefulness, joy, and fulfillment – the essence of a good life – the message is that we can find it in the practice of Judaism.
The essence of a good life is found in this week’s haftara, from the Book of Micah: “Human beings have told you what is good, but what does God require of you? Only to do justly, love mercy and walk humbly with your God.” Micah, who lived during the time of the First Temple, was challenging the behavior of the religious leaders who were outwardly observant but morally corrupt. They told the people that was most important was offering sacrifices, but Micah taught a different message: what God really requires is to live your life in a way that reflects justice, to be compassionate toward other people and not to be so arrogant to believe that your spiritual path is the only one.
In our day too we get mixed messages about what is good. So how can we be clear about what God requires of us? Our tradition offers us many spiritual practices, from prayer to musar work, which can help us see more clearly what really is important. One of my favorite practices is to reflect on the wonderful image attributed to the 4th century Babylonia sage Rava. He imagines that when we die we will need to answer the following questions:
Rava said: At the hour when they bring a person in for judgment, they ask him/her:
(1) Did you conduct your business affairs faithfully?
(2) Did you set aside time to study Torah?
(3) Were you involves with procreation?
(4) Did you look forward to salvation?
(5) Did you debate wisely?
And even so, if “the awe of heaven was your treasure, yes; if not, no.”
(BT Shabbat 31a)
I understand these questions to be challenges for us as we think about what it means to create a life of meaning. First, are you ethical in your interactions with other people? Second, are you connected to a spiritual tradition that not only challenges you to think and to ask questions but also links you to something much bigger than yourself? Third, whether or not you have children, what are you doing to make the world a better place for everyone’s children? Fourth, what are you doing to bring healing and repair to our broken world? Fifth, do you listen and learn from people who have different ideas from yours and are you respectful and civil when you disagree with other people’s point of view? Finally, do you have a sense of awe and gratitude for the blessing of being alive?
The essence of a good life is to be able to truthfully answer "yes" to those questions.