Those are all good suggestions, particularly if you had to be strictly within 3 hours drive time and didn't want to go to an island. Plus, Travelfish recently added coverage on some of those places as well. If it were me though, I'd probably travel a bit more and visit Ko Samet.
The quality of sand and clarity of water (subject to weather of course) on Samet makes the extra distance worthwhile. Go past Had Sai Kaew to Ao Phai or Tubtim, or past Ao Wond Deuan which is busy to beaches further south. You'll probably see some fishing waste washed up on the sand (netting, floats, light bulbs, etc) but that's just normal with the fishing boats and sea currents. Same can apply for islands all over the gulf. At least in front of nicer resorts they usually make an effort to tidy it up.
Another vote for Samet. It takes a bit longer to get to, accommodation is fairly expensive and you pay a rip off NP entrance fee, BUT the sand and sea quality is much nicer than somewhere like Cha Am IMHO.