A few weeks ago I made a three-day trip from Bangkok to Hua Hin. I took the train to Hua Hin. It was a 2-car sort-of-express train from Hua Lumpong Station (accessible via the new subway) and cost 302 THB. The trip took ~ 4 hours, 1 1/2 of which was required to get to and cross the Chao Phra! I took an air con bus back to Bangkok as the train schedule is somewhat limited. I cannot remember how much it cost, maybe ~100 THB? There was a bus leaving Hua Hin every 30 minutes on the hour/half-hour. The bus took ~ 3:30 and made one toilet/food/driver change stop along with ~ 8 stops along the way. I think you can get out pretty much anywhere if you speak with the driver/attendent? The bus terminates at a bus station in Thonburi. Getting from there back to central Bangkok can be challenging. I took a taxi which took 90 min. to get to Sukhumvit (we waited at one light near the Royal Palace for 32 min. I think it was Constitution Day).

I would recommend you take the train if the schedule allows. The Hua Lumpong train station is easy to get to via taxi/subway. My train left Bangkok ~ 7 AM-ish; I cannot find the schedule right now.