I have an 670 and a 3 Zone receiver... anyone have best practice or how-to info on the best/easiest way to set this up?

I think you treat each zone as a device but I'm not positive. Did you do a search? I know it's been discussed before and may be in the harmony website FAQs.

I've been trying to figure out the same exact thing with my Harmony One...

In my case, I have a Denon 3808CI AVR, which supports 3 zones. Each zone is completely independent of the other. Not only can any zone play any input, but each zone's power on/off is completely independent of the others. This allows you to be watching a movie in Zone 1, and then power on Zone 2 and play music there. You can then turn either zone off, without impacting the other. Of course, all of the inputs (DVD, CD, etc) are completely shared.

I was setting up activities like "Watch DVD - Zone 1" and "Listen to Tuner - Zone 2". But this doesn't work: Assume I start watching a DVD in Zone 1. And now assume someone wants to listen to the radio in Zone 2. If you press the "Listen to Tuner - Zone 2" activity, it thinks I've just exited my "Watch DVD" activity and turns off the DVD and powers down the amps in Zone 1. I want the zones to be independent.

Perhaps there is a better way to model this sort of activity? I'm stuck!

Generally what I do when switching zone activites is this,
Upon reaching Zone 2 select the desired activity. When that is done start walking back to zone 1 and in between zones press the zone 1 activity again.

Rdlm, Harmony treats each zone as a different device. You need to use the activity feature "leave unused device on" (which is non-selective), or better yet, create an activity (Watch DVD - Z1 plus Listen to Tuner - Z2) with both zone devices participating.