First I have never been to these places.
This one is the Barramundi
http://www.bangkokfishing.com/This is the catfish, plus a host of others
http://www.bungsamran.com/en/I have talked with a few who have fished the latter and they all caught as many fish as they could handle, lots of sunscreen, hats and sunglasses.
Couple of tips, when you arrive and pass through customs, ignore the taxi touts and head up to the departures level and head to your left as you face the street, at the end of the terminal you will see a line of taxis off loading, they will also be looking for you. Pick one and ask if he is using the meter, if yes, that one is yours tell him where you want to go and you are in. This allows you to avoid the 800 or higher baht fee the bandit cabs charge and the usually long lines the government regulated cabs use. It will help if you have Thai baht as there will be a 100 baht or so toll road fee, which you are expected to pay, So either hit a currency exchange or an ATM in the airport.
Taxis are cheap in Bangkok and plentiful, always ask about the meter. always try to keep enough 20 baht bills to pay your cab, often they will have no change. If you plan on using either a lot both have debit card systems at the customer service windows, that saves trying to figure out how much you need to deposit in the ticket vending machine, just swipe and go. Might also get a subway and skytrain map, they are cheap and easy as well. The Skytrain will get you to many shopping spots and tourist locations. Pratunam Centre, for clothes and Pantip for electronic, both riddled with knock offs as well as real, both fun. Centralworld and Zen are two large shopping centers next door to each other with a sky train stop. Many levels hundreds of shops and a huge food court, easily a day.
Pattaya can be a wild and crazy place and most people stay out of the water. It is trying to grow into ore than just an R&R spot with lots of nice hotels, but as Dudley mention a walk on the wildside down walking street amongst the go go and beer bars is worth the trip. There is a small island off the coast that is relaxing, there are little restaurant bars all along its coast, where you sit in a beach chair under an umbrella ans sip suds, they also have jet skis and the parasail things they tow.
It is going to be hot, we are running over 90 every day now. Diner will be the only time you even approach a dress code a shirt with a collar will usually make the grade in most places.
Dinner cruises on the river are a nice one time thing. You can also get rides in long tail boats if you just want to look around.
Chiang Mai is up country and cooler, so many people head there. The kids are out of school so there will be lots of Thai tourists.
There is a nice beach town across the gulf from Pattaya and about the same distance from Bangkok, Hua Hin. The King has a summer palace there and spends a lot of time there, not as busy as Pattaya.
Last there is a large anti government demonstration planned for the 14th. They are usually well monitored, but can something get large and screw up traffic in and around the demonstration areas. Ask at the hotel if you plan wandering around that day.
Any questions please ask.
