I suggest flying up to Chiang Mai from Bangkok, it's very much quicker than by bus or train. Bangkok has 2 airports, both offering flights to Chiang Mai.
For a beach resort I suggest Hua Hin.... Beautiful place and not an OTT touristy place.... If you choose Hua Hin there is a train from Bangkok....
Another suggestion would be an island called Koh Chang.... Bangkok Airways fly to Trat airport and there is a ferry from Trat to Koh Chang.... Koh Chang is on my bucket list of places to visit....
Have a look on YouTube, there's a lot of vlogs on places to visit in Thailand....
Best advice..... don't exchange any money until you get there, and wait until after you've passed though immigration and collected your luggage.... From the arrivals level at BKK airport head down to the basement level and there is some FX booths down on the ramp down to the Airport Rail-link which offer the best rate.... Superrich have a booth down there.... there a significantly better FX rate down there... Compare the Superrich FX rate to what you can get in the UK and you'll see what I mean....
https://www.superrichthailand.com/#!/en
I first visited Thailand 20 years ago..... I've been married to a Thai for 16 years and she moved over to the UK 3 months after we married.... I love Thailand and we go back there a couple of times a year..... Due to being a carer for my mother, currently we're unable to spend my retirement out there, but hopefully at some point we can.... We have a property out there that is our holiday home, a 3 bedroom bungalow we had built for our retirement.