If I go I'd like to do something more advanced--I dive a lot so don't really want to spend the day on the bus for a hum-drum dive, but if there's somethign really worth doing I'm excited!
Realistically the only option is Pattaya, which is about 2 hours and costs just over 100bt on the bus. As has already been suggested the Hard deep wreck at Samae San is (current permitting) a good dive, as is sharks fin which is a reef dive to a depth of around 26m. Most pattaya shops run trips to Samae San at some time, which is around 40km south from Pattaya.
In Pattaya its self the Khram wreck although an 'artificial' wreck now has plenty of life and again current permitting can be a good dive, the only other dive in the area which in my mind is worth doing is koh Rin North Rock (reef dive to around 18m). Unfortunately due to the distance between these sites it is unlikely that you will find a shop that will do both on the same day so you will usually get one ok dive and the second dive will usually be at koh Larn or Koh Sak which are shallow reef dives where jet ski's, speedboats and tour boats abound, and the diving is really not that good either.
Two things to beware are:
1) Most of the better dive sites in the area are subject to strong currents and as you are only proposing to dive on one day and shops run to a time schedule rather than to suit the tides it is unlikely that you will fall lucky and get the best conditions.
2) You will talk with a shop and make a booking to dive somewhere, only to find the next day without warning you are taken to somewhere different.
Also as you state you are looking for something that is 'really worth doing' so to avoid disapointment I would suggest some in depth research because in my mind nothing in the Pattaya or Samae San area is particularly outstanding, the diving around here is simply something that people do when they are already here for other reasons.
