Problems with Inactivity

My question is basically as to what constitutes skillful and unskillful activity. I noticed I return to unskillful craving-related activities when I am idle. However, I cannot ascertain what is skillful in terms of activities. I would think the application of effort is important in terms of an activity, because I am often contemplating/analyzing ideas, but in a slightly lazy way. Concretely, is idleness bad? For example, I often take public transport, wherein I contemplate certain subjects. But, I often tell myself I should read in public transport instead. I think to me, some contemplative activity occurs when idle, but it is not focused, nor effortful. Can mere contemplation without much effort or focus be called meditation? Or is it simply wasting time? Thank you. EDIT: Also, I would think activity and industriousness generates energy in some sense. What states accompany idleness? (e.g. lethargy, fragmentation, etc.)