If DS won't nap and I'm certain that he needs one, we get in the car and take a drive. It's a short nap (45 min or so), but it's a nap and will of course, stave off the fussies too early in the evening. All the same, I usually put him to bed a little early... I should add here that particularly when DS has a cold, he fights naps because he has trouble nursing (doesn't seem to bother him not to nurse at night). Anyway, during colds, I don't even bother to try and put him down the usual way. We just gather up the sippy and the blanky and go for a ride until the day that the nasal congestion clears.
Sometimes, if he doesn't nap and he seems in good spirits, I just let it go and put him to bed early. Sometimes he's ready so early that it sneaks up on me. So now if this happens, I make sure to have him in a clean diaper and jammies by no later than (and usually earlier than) 6 o'clock... you'd think he'd awaken early in the morning but usually the missed nap catches up with him and he awakes at his usual time (uh, this is not to speak of his usual night wakings... but that's another subject.
Best of luck.