I have travelled to lots of places in South ( mainly Karnataka, Tamilnadu, Kerala), though not specifically in Summers. However you can visit some places like Chikmagalur, Kodaikanal, Coorg which are few of the best hillstations in south to visit during summer. There is also Dandeli, few kms from Bangalore, which is very good for water activities and apt in summers. Also please conduct your own research on temperature, weather currently before making any decisions.