Hello Rohan !
Firstly kiddos that you are actually thinking to explore areas around you all by yourself. There are multiple places you can visit near Mumbai. If you are planning just a single day trip then I would suggest you go for any camping or something. This will even help you in meeting new people and actually explore the excitements and possibilities of traveling solo. If you do have time and thinking of spending 2-3 days, do visit areas around coastline like Tarkarli or Malvan. You can even visit Nashik and enjoy your day at Sula Vineyard.
Do let me know if this is helpful in any way. !! Good luck !
It depends what type of trip you want if you want to go for hiking and camping there are many 2 days treks near Mumbai and also overnight trek
Kalsubai sunrise trek
alang madan Kulang
Irshalgad
Garbett point( near Matheran also you can stay there check my profile)
and therr are many sahaydri treks
if you want to explore places and culture go to
Hampi
Gokarna
Goa Coorg
Udaipur
Bordi Beach (check my profile
mulshi dam(only in rainy season)
Mahabaleshwar
Try going to some unkown village and understand their culture and lifestyle