So I suppose I should detail what we saw in Hua Hin and the surroundings, having thoroughly explored the area in a beat-up rented Vios (and later a Corolla Altis with even more miles on the engine). Since the beach gets all the attention, let's start with the park...
Khao Sam Roi Yot National Park started as a marine park, and some of the sights are best seen from the water. After some haggling using my rudimentary Thai, we caught a couple of boats, one around the islands before landing ashore near the Phraya Nakhon cave, and another down the Kaeo Daeng river.
If this park were in America, every peak would be named; every species of animal lovingly documented. Instead, it's a sleepy national park in Thailand, featuring some tremendous caves - Phraya Nakhon, which was discovered by a Thai king and features an entire throne inside the cave, is spectacular, once you brave the 430m climb to get in. (Our guide, a young woman, naturally hopped up the heights in surefooted mountain-goat-like fashion; we followed panting and pausing to catch our breath.)
Sam Roi Yot also has mangroves and wetlands, with an impressive array of bird life (well, impressive if I were more ornithologically inclined and actually knew the names of the species), but what is really special is simply the view: mountain peaks, either covered in a carpet of lush rainforest or beautifully wizened and craggy, rise up next to the rivers and sea. Sure, the peaks are nowhere near the heights of those in other countries' parks, but they are picturesque, and their tropical setting (you see bougainvillea grow on the mountains; you also see coconut trees at the base) makes them a unique sight.