You can start with Ooty
Kodaikanal Coonoor
Valparai
Topslip
Meghamalai. Coming to some popular spots where monsoon starts setting in august is Kanyakumari Rameshwaram
Madurai
Coimbatore
Hogennakal
Karaikudi Some beach towns to visit in august
Dhanushkodi
Rameshwaram
Tirunelveli Tuticorin
Kanyakumari Other spots that look warm and have small downpours In august are
Srirangam
Thiruvannamalai
Kutralam
Kumbakonam
Chidambaram
Thanjavur ams
Kanchipuram