I have the typical AV setup I'm controlling in my living room, but also have some Air Conditioning units that have a remote control. I'd like to be able to control these units (on/off, temp, etc) without interfering with the other activities.

What's the best way to set this up?

Specifically, I don't want to turn off the AV system when I turn on the AC units and vice versa.

I have set up a "Control AC Units" activity, but now what?

Thanks!

You could do it the other way around: add the AC as an additional device to all the other activities, and allocate (customise) its control commands to spare keys or the screen.

Set up the AC activity but set it to "leave unused devices on" so it won't turn off your HT components. Then you can return to the previous activity.

But the easiest way is to use Device mode - just hit Devices and select the A/C unit and when you're done, hit Devices twice and you're back in the activity without interrupting anything.

I was assuming he didn't want to set the AC to "always on".
Probably though it would be best - just this once! - to ditch the Activities principle.

Like Hhatkin said, you can add the AC device to all activities in one go by using the special option "Select the Activities that use this device" under "Device > Settings" (it's under "Device > More Options" for the website interface). This special option is only available to Home Automation device types (Light Controller, Climate Control, Home Appliance).

If you don't want the activities (including the built-in Off activity) to control the power of the AC device, then simply select the option "There are no power buttons on my current remote for my AC Climate Control" in the Device Adjust power settings.