If you're looking for a relatively inexpensive respite from Bangkok but don't have time to fly to one of Thailand's islands, I would recommend checking out Pranburi. Just south of Hua Hin, it is quiet and chill—not a place to party but a good place for a few days away from it all. However don't expect an amazing beach; book a hotel with a nice pool instead.
Getting to Pranburi by van
I bought a minivan ticket to Pranburi and back through 12goasia, which was easy and super cheap. They ask you to check in one hour early (I’m not sure what happens if you don't), but you can usually wait at a nearby coffee shop. Be aware that when they say there is no place to store luggage in the van, they aren’t kidding. Luggage will have to go in the aisle or in a tiny space below your seat. They fit my carry-on size bag between seats, but if I’d had a larger bag I would have had to pay for an extra seat to store it. This is because they try to fill up every single seat. However the tickets are so cheap that paying for your luggage is probably worth it: I paid 180 baht from Hua Hin to Bangkok for example; less than $6.
They stop once each way for people to use a toilet and buy snacks. Be aware that if the van isn't full when they leave, you will almost certainly stop to pick up passengers along the way until all seats are full.
On the way to Pranburi, the minivan left from the Ekkami BTS station in Bangkok. The counter wasn’t well-marked, but I showed my PDF ticket to one of the ticket sellers who pointed me to the correct desk to check in. I felt a little stressed because there are no announcements made in English at the bus station and I was afraid I’d miss my departure, but I shouldn’t have worried—the person I checked in with came to get me when it was time for our van to leave. We left 30 minutes late and I had to wait in Hua Hin to change to another minivan so it took 5 hours total to get to my hotel in Pranburi (one hour longer than what the ticket had estimated).
On the way back to Bangkok, I took a taxi from my hotel to Hua Hin station, which added to the price but seemed worth it for the convenience. Hua Hin station is much more well-marked and there is a Café Amazon if you want to have a drink while you wait. The bus left on time and only took 4 hours to arrive back at the Ekkami BTS station, so it was a more pleasant experience.
Pranburi itself isn’t much; there’s a small town which I didn’t explore, although it has a few 7-11 shops which come in handy and there’s a market if you want street food. The beach itself was a bit disappointing; it is only located at the southern end and is called Khao Kalok Beach (it shows up on Google maps). The other “beach” which shows up on google maps isn’t actually a beach at all but more of a boardwalk because the tide comes all the way up to the stone steps.
What I liked about Pranburi though is its kilometers-long promenade along the ocean, plus an additional bike path which makes it super easy to walk, run, bike, or just sit and look at the ocean. There's not much traffic so it feels very peaceful. I easily biked six kilometers from my hotel to Khao Kalok Beach and spent a few hours there.
There is also a National Park with hiking and walking trails but I didn't take time to go there; I just enjoyed relaxing and reading a book for two days.
Eating in Pranburi
I tried a few restaurants and didn’t find any standouts, but Chili and the fancy Luna La Pran restaurant at the Sheraton were both pretty good. There's plenty of street food if you want to try eating with the locals. Pran’s Corner was a nice place to sit and have fried calamari and a drink (although be prepared for loud music!). Don’t go to Pranberry; it looks cute but the portions are outrageously small and the service is abysmal.
Sleeping in Pranburi
I stayed at BC Summer Beach, a lovely little hotel with an incredibly friendly manager who doesn’t speak much English but took me to 7-11 on her motorbike when I arrived too late for dinner. The pool is in the shade and covered in pine needles most of the time but that was ok with me. The hotel was quiet, nicely decorated, very comfortable, and has a huge breakfast with both a cold buffet and a hot meal they bring to you. The only issue I had was that they didn’t accept credit cards but as long as you bring cash, I would highly recommend it.