No flight needed. Abu Dhabi sits roughly 90 minutes down the coast by intercity bus or private transfer, close enough for a single overnight add-on without eating into the rest of a trip. Legs a day or two past the marathon handle this better than legs the morning after; both sites here involve a fair amount of standing and walking.
Sheikh Zayed Grand Mosque
Capacity for 41,000 worshippers, white marble throughout, and 82 domes across the complex. Entry is free, but the mosque operates a modest dress code (covering shoulders and knees; headscarves available for women who need one at the entrance) and closes to visitors during prayer times, so checking the day's visiting hours before travelling out saves a wasted trip.
Louvre Abu Dhabi
Jean Nouvel's 2017 building on Saadiyat Island is as much the reason to visit as the collection inside it: a latticed dome that scatters light across the galleries below in a pattern that shifts through the day. The permanent collection spans roughly 5,000 years of human civilisation, arranged thematically rather than by region or era, which makes it a manageable visit rather than an exhaustive one.
Where to Stay and Eat
Saadiyat Island and the Corniche both put a hotel within easy reach of the museum district and a flat waterfront walk, useful for legs still recovering. Abu Dhabi's pace is generally slower and less crowded than Dubai's, which suits a one-night stop built around two specific sights rather than a packed sightseeing schedule.
Getting Back
Intercity buses and private transfers run the Abu Dhabi to Dubai route throughout the day; the return journey takes the same 90 minutes or so as the trip out.