Sigirya would a long day trip from Colombo. You might do better not staying in downtown Colombo, somewhere near the airport might offer a quicker trip, free of urban congestion at least, but I am not sure about the existence of a cross country road from near the airport.

Rates at the Hilton have gone up significantly recently. I am presently at the Cinnamon Grand which has better rates on Orbitz and which is broadly comparable.