Zayed International Airport
Bus A1 reaches Al Zahiyah downtown in about 50 minutes for at most AED 5 on a Hafilat card; night bus N2 takes over after 23:05, and taxis run on the meter.
Every option priced for your party, with where each one leaves you. Straight to: Abu Dhabi Bus Station. Going the other way? Abu Dhabi to Zayed International Airport.