Monsoon is definitely the best time to hike and visit the forts in Sahyadri. The weather is usually cool and one doesn't need to worry about carrying water as there are numerous streams everywhere with plenty of good water. The mountains are green and lush. All the clouds and fog makes the scenery even more enjoyable. Also, the little bit of hiking excludes the kind of crowd (found at Lonavla and Khandala) that you want to get away from. The picture on the left is 'Peth a.k.a. Kothaligad', a perfect day trip. It is just a short bus-ride away from Karjat, the last station served by local trains on Central Railways. One great feature of this fort is the stairs that are carved in stone inside the thumb-shaped tower on top of the mountain. The stairs take you to the topmost point of the fort and view from there is absolutely breathtaking.

The photo below shows Ratangad, near Igatpuri in the northern Sahyadris. It is a little far from Mumbai and makes 2-3 day trip for a comfortable hike. There is a nice big cave on the top to spend a couple of nights in. Beware of monkeys that try to steal food from the hikers.