It depends on what interests you. Also, how far would you like to travel. There are a bunch of crazy options within 100 - 200 Kms that you may want to explore as a solo traveler. "Mahabaleshwar" is a great destination if you love nature. Go for morning walks, bird watching, trek to the waterfalls and the fresh strawberries. Dont worry about the off season. You will still be able to enjoy the delicacies with strawberry flavor. Look up for "Kamshet" if you want to experience adventure. Go paragliding (this is the best destination for beginners). "Karnala" is another destination, less than 100 Kms from Mumbai. It is a great escapade into the nature.
There are so many places depends on where you want to go and what kind of activities you want to perform.
Scenic : Mahabaleshwar, Lonavala, Khandala
There are a lot of places to visit in Mumbai. There are a lot of places to visit near Mumbai too.
If you are a Nature freak you can go for a-one-day trek to Andharban Forests.
140 kms from Mumbai there are Lonavala and Khandala.
Not much of an escalation but these places are amazing.