Hua Hin
I like Phuket, but I wouldn't want to fly there for a really short visit - it's not just the flight time but as we all know the hassle of security, etc.
I think Hua Hin has great beaches. I don't know anything about visiting for a day, but there are 3 good hotels - the Sofitel, Marriott and Hyatt Regency.
Hua Hin is about a 2 1/2 hour drive from Bangkok. There's also a train, but I know nothing about that.