Water festival! Surely that means Loy Kratong at this time of the year....
Bangkok is such an unknown I would never take anything for granted. A projected 30 minute trip could literally take 2 hours. And by the way most hotels are only 30 minutes from the new airport anyway. Even the old airport is only 30 minutes from new airport. Tolls in Bangkok should never cost more than 65 baht from new airport and taxi fares would normally be around 230 baht + 50 baht airport surcharge, only if going from airport, plus tolls. Always ask for highway and they understand.
Just as a reference I travelled from Ding Daeng, close to Victory Monument, to the elephant zoo on the outskirts of Bangkok on what was close to a 1 hour trip for around 240 baht. Do not ever pay more than 250 baht on the meter regardless of where you are going in Bangkok.
And yes I know, I have heard of many times, people who have paid 800-1000 baht for a meter taxi for a simple airport trip to city hotel. That is ludicrous. Up to you....