Travel light: less is more - there is great freedom to be enjoyed in leaving cares and things behind.  But there are a few essentials:

Always:

Water
Money
Inner tubes (2 are recommended)
Tools to remove your wheel and tyre

             (ride leader will help)
Pump (consider a pocket size for enroute)
Lights
Lock (carried on your bike frame)
Puncture repair kit

Consider:
 

Small rucksack or small frame bags for days out              rather than rack & paniers
Helmet

Gloves, or fingerless mitts for warmer days

Waterproof but breathable outer layer

Padded shorts
Wear several light layers, so you can take one                off as you warm up  
Eye protection (sun glasses)
Sun screen (seasonal of course)
Snacks (flapjacks, bananas)
Map (not necessary on led rides)
Camera
Mobile phone
Antiseptic wipes (2-3 individually wrapped)