The only events I've heard described as "can't miss" with respect to the F&WF are the "reserve" or "signature" dinners they have at various times. Most of those, however, are probably sold out.
The good news, however, is that we gladly organize our trips around the festival all the time, and don't feel particuarly left out for having missed those events. The booths and the free wine seminars are enough for us, even though the quality is a notch down (and the prices a couple notches down) from the stuff you'll get with the dinners.
Plus, I believe enough of the ancillary events (F&W pairings, and perhaps the "Party for the Senses" events) still may have some availability.
The Restaurant board has more info on the festival than one could possibly need.