Lonavala is best and nearest hill station to Mumbai. I visited lonavala last year with some of my friends. And I found it very adventures and delight place.
Best thing about lonavala is that it gets more beautiful in rainy season. So I prefer to visit it in monsoon not in summers. Place is nothing in summer just hills. Lonavala is second hill station I visited in my life and it was beautiful and surprising there.
There are many hotels available. You can even rent out pent house for 2-3 days. You can reach Lonavala by private car from MUmbai.
Tips which I would like to give is take an extra water proof bag to keep you belongings safe because rain strikes anytime. And stay away from steep edges. Many people have died falling from the cliff. There is only one word I can describe whole lonavala and it is "BEAUTIFUL"Go and enjoy your vacation