I keep a 32t cassette, a chain for it and a long cage derailleur and swap them onto my roadie for long alps trips. I switch back to normal road gearing for the rest of the season. (I am weak, lazy and a bit of a spinner). I always leave the standard chainrings on the front - can't remember what they are, it's a non-compact double though. I rarely hit the very lowest (32t) gear unless really tired and/or steep road. I do notice the larger gaps between gears.

The dinnerplate sized cassette worked out cheaper and easier than finding small chainrings - it does look a bit strange though.