Yes 7 to 8 hours is plenty of time to visit and have dinner in nearby
places, Also there is a recommendation from official Abu dhabi
Airport guide and places to see in short interval:
http://www.abudhabiairport.ae/english/airport-information/transit-information/more-than-4-hours.aspx
Because I have been an Etihad frequent Flyer for the past 4 years, it is plenty of time to visit close by places. Normally immigration takes 30 to 45 mins, still if you keep 2 hours for immigration(exit + entry) 5 hours is plenty of time . Also catching a taxi is always recommended for short duration.
First I visited Yas Island(took 4 to 5 hours) and another trip I made to Cornich Beach area took around 7 hours
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