I've just got back from riding around the Limousine in Southern France. Under my seat I took - Cable ties (got used), Duck Tape (got used), Ultra seal (got used), Bulb kit (required by law in a lot of Europe), First aid kit (again required by law and the pain killers got used), Electrical butt connectors (Oooh matron), Head torch (got used), Spare clutch lever (don't ask), latex gloves (if you break down they save you from getting oily hands that then have to go back in your riding gloves), small can of chain lube (invaluable), Cargo net and spare fuses.
The guys I was riding with think I had an RAC van under there.
A few other things to think about:
Get a European Health Insurance Card (google it, you can order one online), I know someone who would have a had 100,000 euro hospital bill after a bad crash.
Ask your insurance company for your green card and find out exactly how well you're covered in which countries.
If you're riding in a group nominate a "peage monitor" who holds the groups kitty and pays for everyone at the toll booths, it means you get through so much quicker.
Consider using the "sweeper system" for group rides across country, it saves loosing stragglers.
Most of all have fun, set off the speed cameras and try not to get too depressed when you return to our overcrowded, camera and numpty infested roads.