I usually try to have a single main task that requires my attention, and fill the gaps with menial tasks that don't require a lot of focus, clean up the code, improve documentation, add examples, spell check the wiki, stuff like that. If I run out of things I'll search for some article or tutorial about something related like a library or a tool I'm using and see if there's anything useful in there that I can apply to my code. If that's still not enough I'll consider starting something else or just browse HN for a while if I just need wait something like 10 or 15 minutes.