Regardless of lounges (I have no experience of EY lounges here), I'd would definitely try to see some of the city.

The airport is 20 miles from the city but taxis are very cheap, c $0.75 per mile equiv. Yas Marina is v close to the airport ... newish man-made resort, F1 racing circuit, hotels, mall, etc.

The outstanding grand mosque is half-way to the city. You might make last entry at 9pm or just take some pics from the outside. Then on to the city. What do you like ? Shopping ? Good food ? Beer & hookers ? Or just take a stroll along the Corniche. Visit Marina Mall and take the glass elevator up the 100m sky tower for a good (free) view of the sky line.

Head back to the airport around midnight. On the highway it only takes 30 mins.