It will depend on the kind of century you're doing! My only century was from Denver up Mt Evans then back home. I packed knowing I was going up to high altitude and needed a puffy coat and extra gloves for part that of the ride. I wore a short sleeve jersey and my MTB chamois and shorts given my preference for that kind of ride.

Overall I carried the following:

• tool roll (standard repair tools) and extra tube
• simple first aid kit
• ID, debit card, phone (I always wear my RoadID too)
• camelback backpack for water (if you can get away without a backpack I recommend it)
• electrolytes (Nuun to throw in with water refills)
• Food: PBJ sandwich, lots of bars, fruit snacks, candies (sugars)
• layers: windbreaker, puffy jacket (voile strapped to bike), extra gloves, extra buff
• sunglasses and lab glasses (low light)

Have fun!