We Arizonans understand western fans. But I'm not sure Tombstone is worth 640 miles and a day's travel. You can see it in a few hours. Here are some other things you could see between Lubbock and Tombstone: Carlsbad Caverns, White Sands NM, Silver City NM, Gila Cliff Dwellings. Once you get to Arizona, also visit Bisbee an 1890s mining town much as it was about 20 miles southeast from Tombstone.
Other things you might want to see here include Kartchner Caverns about a half hour from Tombstone. Also go to the Tucson forum, and see Top Questions at top right about things to do which also includes a link to a blog with many more.Edited: 07 February 2013, 05:46
Billy the Kids Grave Fort Sumner NM. Lincoln NM for Billy. Smokey Bear SP Capitan NM.
A word of warning for anyone traveling in Arizona, especially in Sierra Vista: there are red light/speed traffic cameras everywhere. There are not always signs posted to warn you of this. We got a speeding ticket in Sierra Vista from one of the traffic cams as we were going around a vehicle. The ticket was $269. With no way to fight this, you are stuck. And it does go against your driving record. So really watch yourself in Arizona.
Just an update: the speed cameras were outvoted by us residents and have been removed as of 2015.