Abu Dhabi is the capital of the United Arab Emirates, and it plays the part. The city sits on a T-shaped island in the Gulf, and the emirate around it produces about two-thirds of the country's economy. Its oldest stone building, Qasr Al Hosn, went up in 1761 under the supervision of a builder named Mohammed Al Bastaki, and almost everything else you will see is younger than your phone contract. The money bought something different from Dubai's: a mosque, a palace, and a museum district built on an island with the Louvre's name on the door until 2047.
So this is the city for people who want culture more than skyline. It is quieter than Dubai, and the big sights are spread out enough that you will use taxis. Three days is the right length because the highlights fit without rushing. A fourth day would mostly be a beach or a second theme park, and the real extra day belongs to Al Ain.
The plan runs from the old center to the museum island and then to the east, so you cross the city only once. Day one is Qasr Al Hosn, Qasr Al Watan and the Corniche, day two the mangroves, the Zayed National Museum and the Louvre, day three Yas Island and the Sheikh Zayed Grand Mosque. If you are coming from Dubai, our 5 Days in Dubai guide covers the other half of the country, and the notes on the United Arab Emirates are worth reading before you land. Start on a Saturday: the Louvre is closed on Mondays, Qasr Al Hosn opens only in the afternoon on Fridays, and the Grand Mosque splits its Friday hours around prayer.