Get a full set of sae & metric sockets, extensions, ratchets, etc. in a case with a spot for every socket. makes keeping track of them much easier. i have a small craftsman set that i've supplemented w/ various extras ( u-joint sockets, several extensions, ratchet-driven hex drivers, larger metric sockets, etc).
My travel tool box is a Tool Caddy, available from sears. 3 trays, and can function as a stool. Top tray has various crimp connectors, fuses, wire (insulated and bare), plumber's tape, various screws, bolts, nuts, and washers, cotter pins, quick splice chain connectors, a 12v test light, dial calipers, measuring tape, electrical tape, teflon tape, a _small_ first aid kit, a relay or two, a couple of fuse-holders, some 2-part epoxy (both liquid and solid), wire ties, and probably some other stuff I've forgotten. Middle tray has crimping pliers, regular pliers, sliding-adjustable water pump pliers (channel locks), a couple of pry-bars, 2 or 3 flares, a full set of screwdrivers in straight and philips, metric and sae wrenches, a couple of "crescent" wrenches, a breaker bar, vise grips, a set of files, and a hacksaw blade holder. Bottom tray holds multi-meter, 3 hammers (small, medium, and large), socket case, extra case full of extensions, brake fluid, impact driver, wire brush, large (cheap) "rambo" knife in sheath (I used this to cut down a small tree one day, to create a path for a car that ended up in a river bed during an autocross I was driving in), a set of small picks, a spare sparkplug wire, and, I believe, a spare plug. Oh, I've got a butane fired soldering iron & kit in there too.
I've done a LOT of work out of this box, and expect to do much more over the course of my life.
Your tool box may vary
G'dood