Hill stations are most preferable place during summer..it depends on your budget..if you have sufficient time and budget, then I would like to suggest you to visit Meghalaya or Arunachal during summer..
There are many places . Depends upon your starting point and the overall budget. To name a few :
1. Dharamshala , Shimla , Landour , Nainital , Mussoorie , Gulmarg and Patnitop in North India
2 Coorg , Ooty , Yercaud and Kodaikanal and Munnar in South
3 Tawang , shillong , Gangtok and Darjeeling in North East