God considers a good intention as though one had performed a good deed (Kiddushin 40a).
Our time is finite, and therefore every moment is precious and irreplaceable. Yet sometimes we waste precious moments because we do not have something constructive to do.
Some people carry a small book with them, so that if they must wait for a bus or sit in a waiting room, they can use the time productively. It may be a book of Psalms or something to study. It may be a notebook to record an idea or jot down plans. While this practice is excellent, what about the times when we are in bed and cannot fall asleep, or are walking down the street and we cannot read?
An excellent idea is to think about how and when to perform a commandment when the opportunity arises. Plan how we can contribute more to charity or other benevolent deeds, or make mental inventory of the sick, lonely, and needy, and think about how we can bring cheer into their lives. One can reflect on the opening phrase of the Shema, and reassert one's belief in the unity of God and in His providence. One can dedicate oneself to serve God with all one's heart, soul, and might, as we declare when we recite the Shema.
One can also think about self-improvement and how to avoid doing things that one regrets having done. This is part of the commandment to repent, and can be fulfilled at least partially by meditation and concentration.
If one had idle cash and knew that it could be invested for great profit, one would certainly seize the opportunity to do so. We ought to do the same with idle time.