What is the best thing to do in Dalian for a cruise passenger who has 8 hours?

Yes, it is possible. Just inform the cruise staff in advance of your disembarking and get a stay permit upon arrival. Then take a taxi to the town for have fun on your own.

If you would like sightseeing, I recommend the Xinghai Square to you. If you prefer to explore something special in town, the Shengya Sea World is highly recommended. You can spend three hours happy time there.