Going to Bangkok for 3 or 4 days for visa run and would really like to get to a beach, but obviously donāt have too much time. Are there any beaches near Bangkok worth going to? And how about cheapish hotels/hostels/backpackers WITH SWIMMINGPOOLS?
Take a bus (or cab) down to Pattaya, around 1:30 - 2:00 hours drive only and cross over the hill to Jomtien beach.
I wouldnāt dare to go to any beach closer to BKK. Hua Hin on the other side of the bay is also said to be ok though I havenāt been there myself.
No idea about cheap accomodation in Bangkok though ā¦
I went to Hua Hin, it was great. But I was there with my family, we stayed in some sort of resort and had rented a car to do daytrips.
Wait, oops, that was 12 years ago 
No clue what it looks like today.
Iris
Rascal is right. Pattaya is a probably the best beach near Bangkok. Pattaya is also known as a party place. Use to be where the US soldier hung out. So, lots of gogo bars. But watch out for the girley man. Theyāre everywhere.
The bus fare is inexpensive, and it is a good 1hour+ ride. Not a recommended beach, but go if you have to see the shore. Canāt help with the hotelsā¦ But there are tons in Pattaya.
I havenāt been in about 8 years, but it seems like Koh Samet isnāt too much farther away than Pattaya. It used to be a nice place. Anyone been there recently?
Been to Ko Samet about 6 years ago, quite nice. Stayed at a cheapish bungalow with cold water shower (brrrr) right on the beach. 
Think you need to go down to or near Rayong to take a ferry from there to Ko Samet but if time is tight Pattaya / Jomtien beach is probably easier.
In Pattaya you can find many, many small hotels, just walk in and ask to see the room - those days I paid around NT$300 a night.
Had air-con and was clean but canāt recall the name.
Rascal, why do you say that ppl should watch out for āthe girley manā? I also saw a lot of these guys when I was in Thailand, but didnāt see them as a threat. Obviously I was misinformedā¦are they the āgangstersā there? Have you had any bad experiences?
Please let me know as Iām also planning a trip back soonā¦
Samet
Rascal, why do you say that ppl should watch out for āthe girley manā?
I didnāt say that. I do however assume that Rampage wanted to āwarnā male visitors.
You know, the next morning: āWhy are your feet so big?ā ā¦ 
Personally I didnāt feel bothered at any time by the ākatoyā as they are known in Thailand.
KO SAMET. See my post in this related thread: segue.com.tw/3/viewtopic.php?t=7456
According to several Thailand ex-pats I met while living there last year, Pattaya is just another played-out Thailand resort (add Phuket and Ko Samui, even Ko Tao): too many people, expesnive accommodation, noisy watercraft messing up the ecosystem, peddlers, sex tourists, AND, many of the hotels have a bad reputation for dumping sewage right into the water. Yes, people have gotten sick, and not very long ago. Even Lonely Planet gives Pattaya the dis.