I appreciate this topic being addressed as this seems to be a topic faced by most developers at one point or another. Personally, I find that the amount of free-time I have seems to have a direct correlation with my level of productivity and the number of things I'd like to work on. So I have found that keeping a list or backlog of things I'd like to work on is helpful when I find myself with some spare time, as others have suggested.
Though, I still often find myself in a similar situation to what you're describing, so I'm interested to see what solution others have to offer.
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