Taxis only take cash.

There are plenty of kiosks open and you can get money after leaving customs/luggage claim. For best rates, the $100 USD is your best bet. Rate difference between airport and Bangkok city is on the order of 1% to 2% so exchange the minimum of $100.

There's also convenience stores in the airport, post-customs, where you can get beverages/snacks. I usually stop by to grab a soda and salty snacks after the long trek/wait through immigration. It's also a good opportunity to get change, as I often only have large bills left from the last visit. Unscrupulous taxi drivers have been known to pretend not to have change - easier to have the right amount on hand.