Easiest way to reduce power consumption to phone only Sometimes I'm in a situation where I can't charge and want to have phone service as long as possible.  I won't care if nothing else is enabled.

What's the easiest way eliminate as much other power consumption as possible?  Minimal apps, no sync, no data, GPS, Bluetooth.

I'm configured to easily turn off all the radios (BT, Wifi, GPS), but I'd like to strip down as much else that burns power as possible without spending a lot of time reconfiguring.

...AND I'd like to be able to return to normal use just as easily (rebooting would be fine for this, too).

In my case, needs to work on CDMA network, too.
See my related answer about battery conservation.

In your case, you want your phone to become a "dumb" phone.  Use APNDroid to cut off your data.  There's even a widget to make it a one button press.  Other than that, keep your screen dim and you should be good to go.