I'd carry tune up parts: plugs, points, condenser, rotor and
old cap/wires if I have them (new ones belong on the car!), old radiator hoses and fan belt (new ones on car, too). A coil of wire and connectors.
Consumables:
Bulbs.
Oil.
Water. (Odd not to see that mentioned before.)
Brake/Clutch fluid.
Tools:
Combination wrenches: 3/8-7/16, 1/2-9/16, 5/8-11/16;
sockets in the same sizes, Plug Socket.
Screwdrivers: Straight & Pozi and one big one for prying. Standard and Needlenose Pliers.
Vise grips.
Jumper cables.
Multimeter.
Flashlight- the kind that straps to the head is VERY useful.
And the ever popular duct tape and coat hanger wire. Bungy cords can be useful too.
This takes some space but not that much.
I too carry a mobile phone and road-side assistance card, but...
If you're where there's no signal... and if you want to sit for a couple hours 'til they get there....