What does it take for a work day to feel (almost) like a vacation day (without being one, that is)? First of all, it has to be a Friday.
It helps (in my case, living where I do here in the North Bay) if it's late September or early October, when the weather is how I imagine summer in other places. Today was warm without being oppressively hot. The sun was bright but not glaring. The wind was soothing and not chilling. Paradise, for real.
One contributing factor would be that your boss is on the road, in meetings, or anywhere but at his desk where he can reach you instantly by phone or fax. Maybe he checks in once or twice for messages. That's okay. In fact, that's good, because it saves you from having to hunt him down. As long as you don't think about what a lousy driver he is, you don't mind that he's out tearing up the highways.
Oddly (perhaps), it helps if you get some real work done. Not that I work during my vacations, such as they are, but for a work day to feel like vacation, it has to pass without guilt. If I hadn't zipped through the bottom half of my to-do list today, I wouldn't feel nearly as good about things tonight.