Good Lord. For a 55 miler?
Jersey pockets: phone in ziploc bag, coupla Clif bars and/or gels, ID, credit card
Seat bag: same as any other ride - tube, patch kit, C02, multi-tool, surgical gloves, tire levers
On the bike: two water/sports drink bottles, frame pump
Even that feels like overkill, but it's what I bring. Occasionally I'll pack a sandwich if it's a particularly nice day and I've got a sweet stop in mind.