You don't mention an area of the country. Here are three in the eastern U.S.
Marine Corps - end of October, Washington D.C., well organized (it's the Marines, after all), history, monuments, museums. Course is flat once you get past a longish hill at mile 1. Weather usually good. I have a soft spot for this one since it was my first. Registration has been a bit of a crap-shoot the last couple years but they're talking about a lottery in 2014.
Wineglass - beginning of October, Corning, NY area lake and wine country, point-to-point, net downhill but not as flat as some claim. Pretty well organized, good local support. Weather is variable.
Philadelphia - weekend before Thanksgiving, historic city, nice course through downtown, city neighborhoods and park areas. Well organized. About as flat as they come with one moderate hill around mile 9 (unless they change the course). Weather usually OK, but can be on the cool side and might be windy.