It seems to me that there are two kinds of productive days.
There are the days when I come home from work and tell my wife that I wrote 3,000 words on my current project. Those are good days. (Actually, it's more often the case that I emerge from a work-at-home day, which typically involves some time spent writing in a café as well as time working at home, with word counts that high. It's rare that I have such a productive day at the office.)
But there's another kind of productive day, which perhaps only occurs at the office. Today was one of those days. I finished up an overdue map order for d20 Past, started a concept art order for an unannounced book, and in the midst of all that managed to get about 950 words written on my current project (a different unannounced book!). We also had a design team meeting today, where we talked about the introductions to books, siege engines, substitution levels for the book I'm working on, the right amount of psionics content in any given book, and the development process. It was a long day and a busy day, and I really only finished one thing (the map order), but wow, it felt productive.
Not such a productive night, though. I'm intentionally staying up a little late, because I have a sleep study tomorrow night and I want to be tired when I get there (not like last time). So I came out after getting both my son (9:30) and my wife (10:30) to sleep, intending to work on my Eberron novella. Two hours have passed, and I've edited a couple of words. I've also watched several music videos on the iTunes Music Store, surfed every Mac-related web site I know of in hopes of emerging news from Apple Expo Paris (the keynote is supposed to start at 1 A.M. Pacific time—24 minutes from right now), and generally puttered.
Maybe I have only a limited amount of productivity in me for any given day, and I've already exceeded today's allotment. Huh. I doubt it. I think it's more likely that I use the Internet as a way of blocking my own creativity, particularly on the unfamiliar and threatening ground of writing fiction (vs. writing game stuff). So maybe the answer is to design a monster or two before heading off to bed...