The traditional observation of the Sabbath involves, "taking the day off." However, the cultural meaning of that phrase has changed over time. In the context of the social conditions when the idea of Sabbath originated, Sabbath was a sacrifice. The work that people did was tended to be closely linked with their ability to survive, or reproduce themselves as Marx might say. Thus the notion of taking a day off from such work entailed extra work and preparation on the other days in order that a day off from surviving would not kill them.
However, in our modern society work has become socialized into labor, that is, the social image of workers has become people who expect to earn a living wage (enough money to live on, or to support a family on classically) in exchange for selling their work. In this system the connection between taking a day off and sacrifice is nearly erased. Because the social expectation is that employers will pay employees "enough to live on" in exchange for the "employee's work," it is to the employee's benefit for the socially defined amount of work that is considered "full time" to be minimized. By providing rational for taking a day of from labor, the Sabbath is prima facie a benefit to the laborer.
Of course, the laborer could still reproduce the sacrifice originally intended by the concept of Sabbath by devoting the day to God, rather than her or his own personal pursuits, however I believe that this alteration still fails to capture the spirit of Sabbath for two reasons. The first, rather punctilious objection is that, in some sense, the standard religious line is that one's life ought be devoted to God. However, the meaning behind my words can be discerned in the example of sitting listening to a sermon on a sunny spring day, rather than going out to the church lawn and playing a game of frisbee. While there is nothing inherently wrong with playing frisbee, by sitting through the sermon one achieves a sense of self-denial in the name of God.
This brings me to my second objection. I do not believe that God would have us deny ourselves for a large portion of our lives purely to devote ourselves to God. Of course, when our desires are for harmful things, I do believe God wants us to practice long term self-denial. But in the case of harmless personal passions, which we can represent by the game of frisbee, I believe that God would have us develop our own passions and pursuits. After all, did Jesus not say that he came that we might have life, and have it in the fullest? According to John 10:10, he did indeed.
In addition to the meaning of work, another difference between the context of Sabbath historically and in modern society is the amount of work that workers are expected to do. Historians believe that the medieval peasant actually worked fewer hours throughout the year than the modern full time laborer. Thus, refusing to pursue personal development on the Sabbath presents a much greater reduction in the modern worker's ability to express him or herself than it does for a historical equivalent worker.
In conclusion, I encourage a mindfulness toward God in all one's activities. However, short of achieving the ideal Utopian communist revolution, it seems spending an entire day in the devotion for God at the expense of self growth represents a stunting of one's self that is not in keeping with Jesus' desire for us. Note that people who feel that devoting their life to explicitly godly labor is their personal calling can simultaneously devote their day to God and grow their own self, so please do not read this as a condemnation of monastic service. What the Sabbath does seem posed to do is provide a regular reminder that we ought to be living our lives mindful of God's presence in it.