When this sort of thing happens to me it is usually because either I didn't have a concrete plan on what I was going to do beyond describing it as "useful" (or the project wasn't really interesting), or I had been deprived of relaxation due to being very busy for more than a week and the mind is "reclaiming" its time.

If the reason is latter, I wouldn't worry too much. But in order to make my weekends or off days productive, I try to sneak in a small share of web browsing on working days - it basically works like rest/sleep. As for former, I let the interest or necessity generate naturally and not force myself to do something useful and think its a crime to relax.