Day trip to Downtown Shanghai, what to see?I have 18hour layover in Pudong Intl. Airport.Transport?

Yes, you can use these hours to tour around downtown area.
Upon landing, you are suggested to take Maglev Train to Longyang Road Station. The ticket fare is CNY 50 per person. Then transfer to subway line 2 to People's Square, Exit 2. It costs CNY 4 per person. After that, walk southeast around 880 yards (800 meters) to find sightseeing bus. You can take it. It passes many popular scenic areas, such as Nanjing Road, Bund, Garden Bridge, Huangpu River and Oriental Pearl TV Tower.

After that, you can go to East Nanjing Road and taste nice local snacks.
Generally speaking, you need to go back to the airport 1.5 to 2 hours in advance. So about the return way, you can just take a taxi. The cab fare is CNY 180 to 190.