Here's what I carry in my tool bag, which includes tools to fix trailer problems:
1/2" drive ratchet w/sockets to fit the trailer lugs, 3/4" (pylon bolts), 9/16" (trailer bolts - Trailex trailer) and 1/2" (dolphin striker)
3/8" drive ratchet w/full set of sockets from 1/4" to 9/16"
Crescent wrenches - small one for the nuts on the blocks and cleats, 2 1/2" (for the rudder bolts), 7/16" (for tiller connectors, tiller extension swivel)
Screwdrivers - flat bladed - large (for rudder kick-up adjustment and bow tangs), medium, small and stubbie.
Screwdrivers - Phillips head - 8" #2, 6" #1 and #2 stubbie
Dead blow hammer (for beating on castings to get the boat apart)
12 oz carpenter's hammer (for beating on everything else)
12" Half-round wood rasp
Hacksaw blade holder (uses partial blades)
T-handled 1/4" allen wrench (that's for the 17)
1" cold chisel (for knocking the heads off pop rivets)
Channel-lock pliers, large and small
Heavy-duty wire cutters (strong enough to cut a shroud)
Electrical wire stripper / cutter
Hog-ring pliers (for seizing bungee cords)
Surgical forceps (for grabbing little parts)
A "Big M" accordion-style pop rivet tool
Silicone seal, Lock-tite, electrical tape, duct tape, zip ties.
All of this fits into a canvas tool bag from West Marine.
On the trailer, I have an old milk crate that has McLube, a small can of acetone and various cleaning supplies (soap, sponge, brush, towels and a squirt bottle full of water (handy for flushing sand out of traveler tracks)). There's also a bucket full of odd bits of line and bungee cord.
Sometimes, I'll carry a cordless drill in the sailbox, but only when I know I'm going to a remote/unknown location for a few days.
Then there's the "Hobie Hell Box" - a fishing tackle box full of miscellaneous parts from 40 years of sailing and racing Hobie 16s. For the beginner, pins, rings, mast bearing chips extra mast step link, rudder cams and a few shackles would suffice. Extra bits of wire, wire connectors, bulbs and a replacement flat-4 connector will take care of trailer light problems.
I also have a "marlinspike seamanship" ditty bag with whipping twine, sharp scissors, knife, electrical tape, two Sharpies (black and silver), small fids and a pusher.
On the boat, in addition to the electronics, I carry a Leatherman Tool, pins, rings, shackles, a few bits of small no-stretch line and bungee cord and a mast step link.
The point of all this stuff is - you don't want to cut short a day's sailing because a $5 part broke or got lost. In my case, I'm often racing in areas distant from replacement parts. Being able to make repairs quickly can mean the difference between winning the regatta or going home early.