Wear padded shorts and gloves, short sleeved jersey or base layer.
Carry long-sleeved jersey, close fitting leggings, windbreaker plus a waterproof if the forecast suggests. Check the temp forecast for the end of the day and ensure you have sufficient clothing, its really hard when you are tired and cold.
Start fully hydrated by chugging some liquid 1/2 hr before the ride and take 1.5l of water per person (assuming refills). If you use gatoriad, take another bottle of just plain water.
Some food, not neccessarily athletic energy food, just a good picnic with a slow energy release, eg ripe bananas, wholemeal bread sandwitches. You need to snack every hour esp at the begining. A couple of cereal bars are useful.
Spare inner tube, puncture repair kit, pump , basic tools for tightening bolts, adjusting screws, removing wheels.
Camera, map.
If you are assuming an hr of daylight by the end, take at least a rear blinkie light.
For a one-off ride you can bungie a small backpack to the top of a rack. The current fashion for racks that clamp to the seatpost is OK for race bikes but if your bike has suitable threaded eyelets then get a rack that fits them. I like the Blackburn style of rack, light, strong and simple.