Lonavala's defining product and the thing most visitors leave with: chikki, a hard brittle made from peanuts (or cashews, almonds, sesame, or coconut) bound with jaggery — unrefined cane sugar. The result is a dense, sweet-crunchy slab that keeps well and travels without refrigeration, making it the ideal gift-from-the-hills. Dozens of variants exist — peanut is the standard, cashew is the premium version — and most shops also sell a softer fudge (more like a dense toffee) in chocolate and other flavours.
The main street is almost entirely shops selling some version of this, with aggressive touts outside. Long-established names like Maganlal Chikki and Cooper's have built reputations for consistency and hygiene. Both offer samples freely and ship to Mumbai and Pune for those who want more than they can carry.
Best approached as takeaway shopping rather than a sit-down experience. Buy a few varieties for comparison; the quality difference between producers is real.