I have been to Sikkim many times and it is the most mythical yet unexplored regions in the North east side of our country. The Himalayas and the Teesta river, cascading waterfalls and beautiful valleys are definitely perfect for a summer season trip.
Mahabaleshwar is an ideal retreat from the summer heat. Located in the western ghats it will give you breathtaking vies on the valleys from different angles. You can enjoy boating, horse riding, trekking there.
Another destination that is worth visiting is Kodaikanal in Tamil Nadu. The slightly foggy, pleasant weather, lush green forests, verdant hills and meandering roads, waterfalls and viewpoints make it a beautiful place to soak in the serenity.