So whats in your flight bag...and aircraft? Below is a basic list of things that you might want to consider having with you.
VFR maps (IFR charts if you are IFR rated and plan on filing)
At least one good headset (passengers can have earplugs or cheap headsets)
Earplugs (a number of those foam squishy type that the passengers can keep)
A fuel tester (you can never be too trusting of the transient gas truck)
Multi–tool (Leatherman or Buck Tool type of device)
Sick sacks, more than one! (For sick passengers or pilot and portable rest stops)
Motion sickness tablets or pills.
A backup handheld transceiver
Pens, pencil and notepad of paper
A laminated US map and dry marker
Kneeboard (I rarely use, but I have it)
A handheld GPS (hiking works as long as used in conjunction with a sectional)
A four place portable intercom
A portable CD player (or mp3 player) with the input cables for the intercom
Power plugs for all the devices
A multi plug adapter to run more than one device on the power plug (cigar lighter)
Batteries for all the devices
Wire, assorted nuts, bolts and screws.
Extra gloves (I always forget a pair on one trip a year)