The only really good food in Abu Dhabi is at Zuma, which unfortunately is on the other other side of the island, about the furthest point from the airport.

There are ok ones but to be honest its not really worht leaving the terminal if you only have 6 hours just for the food. On the other hand if you want to get out and just maybe relax, try one of the hotels on Yas Island. There is a decent Mexican in the Park Inn, which suprisingly doesnt do burritos but the rest is ok.