Any possibility to travel to Shenzhen for day tour and return same day to Guangzhou?

Yes, it is possible. There are bullet trains running between Guangzhou and Shenzhen frequently daily which only takes about 1.5 hours. You could do this tour in one day.

From Guangzhou to Shenzhen by train costs RMB80 and with return journey will be RMB160. If you have 4 persons but less than 7 people, you'd better rent a private 7-seater car that costs you between RMB800-1000. Usually the driver will wait for you as well as drive around Shenzhen for 5 hrs, and then back to Guangzhou.
Wan