Citizens from many countries are granted a "Visa on Arrival" in Thailand for stays up to 30 days. There is talk that this might change to 15 days, so please be sure and check this closer to the travel time. There is also talk of a digital  Arrival Card which will be completed prior to travel. Please check the Thai Embassy website to confirm you are eligible and/or investigate Visa/travel  requirements.

If you are arriving in Thailand via Bangkok, fastest and easiest is to fly to Krabi.

Please be advised there are two airports in Bangkok.

Most international flights arrive in Suvarnabhumi International Airport. 

Many domestic flights in Thailand  are offered from Don Mueang International Airport.

So keep this in mind if booking an onward flight to Krabi.

Currently there is a shuttle bus between airports but factor in time/traffic if trying to connect. (And this is of course subject to change)

Allow yourself time to clear Customs & Immigration before an ongoing flight.

There is also the possibility of arriving via Ao Nang. 

You can travel from Phuket, Koh Lanta, Koh PhiPhi  via ferry to Ao Nang, making for an adventurous arrival.

There are many ferry and transport companies, but here is one to give you some ideas. 

I would recommend a night or two in Bangkok or somewhere--- ahead of the retreat to acclimatize and overcome your jet lag.