Abu Dhabi is much bigger than first-time visitors expect and its sights are scattered across several islands and districts. The unmissable two are the Sheikh Zayed Grand Mosque and the Louvre Abu Dhabi; if you only have a day, go to those. With more time, work outward from the central island — the Corniche walk, the heritage district at Qasr Al Hosn, the Etihad Towers viewpoint — and then take a half-day each on Saadiyat and Yas islands.
The city is hot for half the year, walking distances are deceptive, and most movement happens by car. Plan around the heat: indoor and air-conditioned attractions in the middle of the day, outdoor walks at sunrise or after sunset.