I've traveled to the cultural center of Sri Lanka, and think that is much too ambitious an undertaking for a day trip. It takes well over an hour just to get out of Colombo before you even begin your trip. It's another 3 hours or so to Sigirya - and then you need a couple hours to hike the 1400 steps to the top. A more realistic day trip is to go to the Elephant Orphanage (be sure to be there when the elephants are taken to the river for their morning bath) and Kandy, which has a variety of sights.

Are you considering staying at the Colombo Hilton? If so, you might email and ask how much they charge for their car and driver. I used Jetwings for my travels inside Sri Lanka. They are a superb agency, but I am not sure they will quote a car and driver for a day. Still, there's no downside in sending them a message.

It's possible to take the train to Kandy, but then you miss the Elephant Orphanage. The opportunity to sit and have a cup of tea on a balcony overlooking the river while 40 elephants bathed was magical.