Lonavala is safe per say but you need to take reasonable precautions as any other travel.
One location you'll probably be planning to visit is the Bushy dam which often gets over-flooded in the monsoons, so travel there but listen to advice locals might give because they know the area better. Also, there's risk of landslides and rockfalls, but with caution, you'll have a great time there. Have fun travelling.
Hey, Lonavala is a perfect place to visit in monsoon you can explore various near by places like "kataldhar waterfall, lohagadh fort" and so many other trekking spots.
all you have to take care of your health as its heavy monsoon so keep raincoat, umbrella and waterproof bags and footwear. Cheers,
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