In case it's not obvious...if you change some money at the airport, it is likely that you will receive a healthy mix of bills and coins, suitable which along with change from the toll-booths, should adequately cover the taxi fare. In the event you require additional smaller denominations, simply request this at the exchange booth.

If you use an ATM (there will be a fee of 150-200 baht) you could even request an amount like, 14,900, which would yield 14 one-thousand baht notes, one five-hundred baht note and four one-hundred baht notes.

Again, don't be too concerned about having change as your destination is likely a hotel, so in the event the driver is un-willing/able to provide change, you could ask him to wait while get change inside the hotel.