I'm on this flight a few times a year (next one in December).

As others have said, the check in is 3 hours before, so once you have done some shopping and used the lounge, there's not much time left. Also, from experience, leave the lounge a little earlier than the call for the flight as the gate is a mile away and you have to go through security screening again, which can cause queues.

On my last day, I tend to leave the bags with the hotel and relax by the pool for a bit before going to Dubai Mall for some last minute shopping. Then I go to Wafi Gourmet for dinner (also in Dubai Mall) overlooking the fountains at Waterfront Promenade. If you haven't been, then maybe also consider going up the Burj Khalifa (entrance also via Dubai Mall).