I have considered getting a steinberger spirit for this exact reason. Tiny, lightweight and to only play it if I really have to. That said, I also want to enjoy playing my bass purchases, whatever the budget. My £299 JB-2 has now become my go-to bass, above my considerably more expensive Attitude LTD2. My Squier 50s P meanwhile seems to have found itself in that unplayed backup bass slot. I'm not sure exactly why. It's a nice instrument, but I haven't bonded with it as much I have other instruments.
Otherwise, I've also been considering getting a Gretsch Junior Jet as my backup. Again, small, light and easy to play. But prettier too look at than the steinberger, and more likely to get played.