Hi Meetu! you're going to have a wonderful time!
Personally, I think Siena is a must-see. You can see the main sights in a few hours, but you could also spend a day. It's a town with NO SHADE so be prepared for the sun to zap you and make you more tired than you should rightly be. :)
Last May some friends and I did four hill towns in a day: Montalcino, Pienza, Montepulciano and Sarteano/Chiusi. The two "M" towns are a must if you like wine -- lots of little vineyards. However, Pienza was our hands-down favorite hill town. There's one main "drag" of shops to pop in and out of, sampling pecorino cheese (this is where pecorino comes from - it's great to buy some and take it with you for your next stop). I can't say there's tons of things to "do" but it is such a charming place!
Finally, I'd recommend Volterra. It's The Town That Tourism Forgot. Great ruins, great views, and a great big park that's great for just relaxing, picnicking and soaking up the local atmosphere.