Bhushi Dam, a few kilometres outside town, is Lonavala's most popular monsoon gathering point. Built in the late 19th century to supply water to the railway workshops, it transforms during the rains into a social spectacle: as the reservoir fills and water spills over the dam steps, crowds of visitors wade knee-deep in the overflow, splashing and photographing themselves against the rushing water. It is less a sight than a happening — and for that reason, on peak monsoon weekends (July–August), it is extraordinarily crowded.
Lonavala Lake, a short distance away, is quieter and offers a more contemplative view of the same landscape. The two are often done together on a short circuit from town.
If you want the full monsoon experience — crowds, vendors, overflowing dam, the particular festive energy of Mumbai and Pune families escaping the city — come on a weekend in July or August. If you want to actually see the place without the scrum, come on a weekday or outside peak monsoon.