In travels to over fifty countries, Sri Lanka remains among my very favourites. In fact, it's the only place I've been back to that was better than I remembered.
Will you have your own vehicle? I highly recommend renting a car, without a driver if possible. Public transport is, or was, poor. I used a motorcycle on both my trips and it was very satisfactory. Sri Lanka is a motorcycle paradise, traffic notwithstanding.
Sigirya (definitely make the climb) should be on your list. As well, Horton Plains and World's End (stay in the guest house at Horton Plains and dress warm) are worth the journey. The towns of Nuwara Eliya, Haputale and Bandarawela are nearby and they'll show you the definition of a "salubrious climate" Perpetual springtime.
Visit a "tea factory" and some tea plantations. There are excellent photo ops with the tea pickers and tea rivals rice as a photogenic crop.
There are several National Parks if you're in search of wildlife photography. For me, the ancient cities that you've already planned to visit, the landscape and the culture are the reasons to go.
"Guest houses" are everywhere and they provided economical, basic housing and a chance to engage with the local economy.
The southern beaches were incredible, but I imagine that the tsunami of 04 has changed all that.
Sri Lanka isn't equipment-friendly. It's dusty, dirty and humid. Beware rapid temperature changes that promote condensation if you stay in air conditioned premises.
Here's just one reason why I love Sri Lanka: In downtown Colombo, my screw-mount (Pentax) 85mm lens dropped off the camera, bounced off the motorcycle gas tank and disappeared into the heavy traffic behind me. I couldn't stop and I gave it up as gone forever. A few minutes later, stalled in traffic, two small boys appeared at my side, gasping for breath. "Sir!" was all they could manage. One of them held the lens, the other the lens cap. Dented, it still worked perfectly. Decades later, I still have it.
Have fun. I wish I was back there myself.
