Matheran is one of the cheapest and shortest weekend getaways from Mumbai. The hill station becomes one of the most breathtaking places in Western Ghats in monsoons.
You can have a short one day trip, go for local sightseeing, do a trek, take a ride on a horse, just enjoy in the toy train or you can stay overnight in there roam around on foot in the middle of night, feel ghostly chills or freak out in darkness. After a point no vehicles are allowed you can take a horse or walk on foot. Make sure to wear comfortable shoes and never new, they will get definitely get ruined in the mud from the rains.(P.S. Only if you are going in Monsoons).