Heya,
Colombo is a big, crowded and polluted city like many in developing countries, however you won't see the kind of abject poverty as you would in say the slums of India, and correspondingly the hygiene is also much better. Mostly the things to do in the city would be to find yourself a nice luxury hotel and enjoy the tropical weather sitting by a pool, and have a Sri Lankan curry at many of the massive hotel buffets available for dirt cheap. There's shopping to be had, and perhaps you can wander to the National Museum, or the Zoo etc and enjoy a visit to a Buddhist temple (remember to take your shoes off).
Now, the real beauty of the country is outside Colombo. If you do have a few days up your sleeve, ask the hotel to supply an English speaking driver to take you to some of the World Heritage sites such as Sigiriya (a rock fortress you can climb) or Anuradhapura, once the capital and where you can see ancient ruins thousands of years old. As you head out to these, no doubt you'll see a myriad of rice paddies, and coconut/rubber farms and plenty of lush jungle, with the elevated parts affording some rather spectacular views.
If you have the time, head to the hill country of Nuwera Eliya to see the tea plantations in a cool temperate climate (as that Dilmah chap would say "do try it") and perhaps round out your trip by dropping into Kandy on the way back, which was the seat of the last Kingdom to fall under colonialism when the British managed to get up there.
Cheers,
Mal
