First decide weather you want to visit in south or north India. There are various adventurous activities in the month of August in jaisalmer, jaipur like camel ride, bir billing whereas you can try sea sports like banana rides , scuba diving at beaches of andaman and nicobar, Puducherry etc. Moreover you can also try famous kedarnath, badrinath treks in uttrakhand which opens generally till August only.
I will suggest go to Kashmir great lakes trek. most beautiful trek in India. it starts from sonmarg, around 90 km, it will take 7 days and covers 7 lakes. Vishansar, Kishansar, Gadsar, Satsar, Gangbal.
Try Sandakphu trek which starts from Manaybhanjan.. Manaybhanjan is 4 hr drive from Siliguri or Bagdogra Airport..in between u can visit Mirik.... Trek starts from manaybhanjan via Chitre, Tumling, Gairibas, KalaPokhari , Meghma n finally Sandakphu.. It crosses Sangalila National Park and From Sandakphu u can see all 7 peaks of Himalaya on a sunny day.. Sangalila is the only national park reserve of Red Panda in India stay at Home stays in Tumling n Kalapokhari option 2 Siliguri to Kalimpong to Aritar to Zuluk to old Baba Mandir ( Harbhajan Singh Temple, Baba Top) to Nathula Pass to Tsongo lake to Gangtok to Martam to Siliguri Road Trip stay at Kalimpong, Aritar , Zuluk n Martam both options will be filled with Adventure n a life time experience.. Option 3 Siliguri to Gangtok to Lachung to Lachen to Yumgthang to Gurudongmar n return..