I keep a small box of electronic parts in the car, for times when I get an idea I want to try out, or have a quick gizmo that I need to solve a problem. I also have a box of mechanical parts, like strings and pulleys and weights, that I'll use sometimes. When I hang out with my engineering friends, we'll often come up with ideas that we can quickly build and test. It seems that I should put together a similar set of LEGO parts.
I'm looking at a target volume about 3200 cm³ (200 cubic inches, 3.4 quarts), including space for organization. No individual element should be larger than 140mm×230mm (17×23 studs). The most important goal is versatility. I'm mostly orienting this towards engineering tasks, but may also throw in a few minifigs for decorative builds.
The best idea I currently have is to use the Creative Suitcase, and augment it with parts from one of the smaller Technic kits that's almost entirely general-purpose Technic parts, like the Compact Tracked Loader.
It seems likely that people have built similar "on-the-go" kits, or have their "hotlist" of LEGO parts that are always at hand when prototyping. Any thoughts on the matter?