Monsoon is Perfect time for mahabaleshwar..the ghats will be all green and beautiful waterfalls around..stay at airbnb or homestays which has good views..read reviews and book accordingly
From Sept to Dec... in summer also best time because of hill station you will get cold breeze up... But yes recommend in Sep to Dec as you will see greenery around and beautiful clouds...
If you are looking for some adventure and fun you must check out Bhandardara, dist. Ahmednagar, Maharashtra. It's an amazingly beautiful place. There are mountains all around. Bhandardara is situated in the midst of Sahyadri range. You must visit this place once at least. Check out The Land Of Wanderlust (TLOW) website..... https://thelandofwanderlust.com/ For Details please share your mail id or what's app message on +918450977725.
Warm Regards