The best ways conventional ways I know of to lower fun are to work on a novel too long, probably three to four hours or more (I've always hated that that doesn't raise fun, but just the opposite is true), or to spend an entire day working too hard at work. Maybe not an entire day, actually, since that will bring on some major negative moodlet action, but maybe a business as usual for a few hours, then work hard for the rest of the day.