In summer, best places to be at are mountains and Himachal Pradesh is the best choice when it comes to mountains & tourism. Manali, Shimla and Dharamshala are best tourist spots in H.P., however, you can visit Palampur, Kullu, The valley of Flowers as well. Apart from H.P., you can also visit Kashmir, Ladakh, Darjeeling, Sikkim, Mussoorie.
Mumbai has a lot to places to hangout at, like, Marine Drive, Worli Seaface, Juhu Beach, Sanjay Gandhi National park, Essel World , Adlabs Imagica between Mumbai and Pune, Water Kingdom, ISKON Temple, Gateway of India, Nehru Science Centre, Siddhivinayak Temple, Tajmahal Hotel, Vihar lake. If you would prefer some lounge, you can consider Nariman Point, Colaba, Marine Drive and Bandra Band Stand.