using speakers without broadband internet.

Sonos only needs a wifi connection once set up, so a mobile internet connection on a phone won't do. Without wifi you cannot connect to your speakers. So you need to connect your speakers and your phone to the same wifi-network, even without internet.

It is important to keep the system on the same version with the app, so, since the systems won't get updates (no internet), turn off automatic updates for the apps on your phone too.

You could use a travel router. 

If you get a travel router, set it up and take it (speaker and controller too) to a friend who has a supported Internet connection you could do the setup there.

Bring a 10 foot Ethernet cable and a pizza and you should be all set.

A bit risky to just take one Sonos though as if you hit an update you may end up making a second trip.

Just wanna use the speakers as speakers? 

Speakers for what?  What source do you wish to use? 

Sonos speakers can not be used to directly replace traditional passive speakers connected to an amplifier.

Of course not.  Sonos speakers are not passive in any way.  Quite the opposite, actually.

If you set up a Five or Play:5 to auto-play LineIn you can. Don’t know if the Eras work the same way? 

But surely you’re rather losing out on “why buy Sonos” if you want to run without internet?

Even that’s not passive.  A traditional amplifier would need a line-level converter to “power” that configuration.

Agreed, but still can be used standalone, which was how I interpreted the question asked.