I confess, I have a rather over-the-top approach to tools and spares, but I only carry it when on a long trip... not day rides.
--screwdrivers, allen set, select sockets, pliers
--wire snips, small voltmeter, electrical tape and your choice of splicing bits, small coil of wire (large and small gauge)
--tire repair kit, pressure gauge and pump
--spare fluids; oil, and brake fluid
--spare fuses for everything, bulbs
--flashlight, ePIRB, multi-tool, spare batteries for everything
If you have a complete kit, you'll never need it. If you don't... ;-)
FWIW, I think a good approach is to have TWO kits... one for short rides and one for LD where you have to assume you may be on your own. Form follows function ;-)