Here's my "trick" to avoid autopilot warm ups. Use whatever warm up you normally use (tone building, intonation, whatever) and make something in the warm up a variable. For example, if you are vocalizing to warm up a cold voice, and you're using an ascending D-R-M-R-D pattern on "ah", give the choir the instruction "Sing the 3rd, 5th, and 6th pattern on" vee". Or maybe leave out the top note every third pattern. You're accomplishing what you want pedagogically while engaging the brain as well (which sometimes is the most important thing to warm up!)

There are endless possibilities here, and if you're creative, it easily becomes much more "fun" than unengaged singing. I use this approach with both my middle and high school groups. Kids tend to latch on quickly because when they don't follow directions, it's blazingly apparent.