The original question sounds (to me) like the search is for something as nearly like a piano as possible, except portable.

There is a wide variety of keyboards available, and with a little searching and testing it is likely that you can find a keyboard that will come close to what you're asking. The next question will be whether you can/want to afford it, since the ones with the "nice" features become more expensive as each feature is tacked on.

For most keyboards in the affordable/portable range, you'll likely find them more like playing an organ than like a piano; and it is something of a different thing. A piano can get some difference in volume as you whack it harder, and you can use the sustain pedal to draw out a note; but an organ, or keyboard of ordinary kind, sustains the note as long as the key is held down. It "needs" a different technique.

Keyboards with "velocity sensitive" keys can respond with variations in loudness in response to how vigorously you attack the keys, but the sustain is still there in all of them I've seen. There are variants with "pressure sensitive" keys as well, I'm told; but I presume still with "organ like" sustain.

I've seen ads for keyboards that make the claim that they mimick the "percussion" response of a piano pretty well, and your digital piano might be of this kind; but the ones advertised were far beyond - in price alone - where my interest disappeared before I did much investigating. If one were to deliberately look for something of this kind, there quite likely are more affordable ones "somewhere out there." Perhaps Rockhen's suggestion of the Casio Privia would be a good starting place, as it claims "hammer action" keyboards.

If you want real portability, it may be necessary to sacrifice an octave or two, and some of the "pianoness." There should also be at least a few that can be run off (internal?) batteries if avoiding the search for a socket is helpful. The actual acoustical power needed to compete with most session instruments isn't all that great, and a modestly sized battery pack should let you play for a few hours before requiring a recharge with at least some of the available keyboards; and if the pack is small enough you can carry a spare.

(The Casio mentioned indicates 12 to 18W + 2 8W speakers, which would be a bit of a load - but not impossible - for a midling sized motorcycle battery for a couple of hours. An extension cord would be a somewhat lighter to carry.)

Almost any digital keyboard will offer a choice of "voices" so if you wish you probably can play the keyboard and sound a lot like an accordion - or a penny whistle.

I wouldn't discourage you from looking at other instruments more "typical" of session playing, as the "piano" can have a tendency to overwhelm some other popular instruments. We'll assume you'll be nice about it ... - ... and at any rate you'll likely be no worse than the squeezers.

John