We are flying to Europe via Guangzhou and Have an 18 hour layover.

John, you can surely have a short tour of GZ.
To Baiyun Mountain: take subway line 3 from the airport to Jiahewanggang, then transfer to line 2 to Jiangxia, Exit D. Take bus no. 664 from Jiangxia to the west gate of the mountain via 5 stops.
I guess you must refer to Shamian Island. If so, you can take bus no. 38 from the west gate of Baiyun Mountain to Wenhua Gongyuan Station. After alighting, take a walk westwards along Liu'ersan Road and the Shamian Island is on your south.
Afterwards, find your way to Huangsha Station to take subway line 6 to Airport South of line 3 with a transfer at Yantang.