How can I get from the airport early morning to the river cruise for part of a day only and back to the airport?

You can take subway line 2 and get off at Nanjingdonglu Station. Then get out of Exit 1 and walk along Nanjingdonglu to reach the Bund. Then walk to Zhongshandongyilubeijinglu station (in front of Yangzi Insurance Building) and take bus no. 868 to get off at Zhongshandongerluxinkaihelu. Then walk to Shiliupu Wharf to cruise on the Yangtze River.

For your return trip to the airport, you can take bus no. 868 to reach Zhongshandongyilubeijinglu and then walk to Nanjingdonglu station to take subway line 2.