I'm planning to visit Hong Kong April 11th to the 15th. then travel to Guangzhou April 15th. to 17th. then back to Shenzhen April 17th to 21st.

For HK, Ngong Ping 360, Big Buddha, Citygate for shopping, Ocean Park, the Peak, the Star of Avenue, Harbour City, Causewa Bay and Temple Street Night Market are worth seeing.
For Guangzhou, Pearl River Cruise, Yuexiu Park, Ancestral Temple of the Chen Family, Shamia Island, Guangdong Provincial Museum and White Cloud Mountain are good places.
For SZ, Dongmen Pedestrian, Window of World, Splendid China, Da Meisha and Xiao Meisha Scenic Area and Minsk World are recommended for you.