Most tourists don't visit Colombo at all and after arriving at Sri Lanka go directly to the south coast, to Kandy or Anuradhapura. We had a car and a few hours of free time, so we traveled around the city.
In Colombo we liked two places: a large square and waterfront Galle Face Green with views of the city's first colonial Galle Face hotel and World Trade Center. Many famous guests stayed in that hotel. On the other side of the square is an old parliament building, where the government was located from 1972 to 1982.
Another interesting place is the Beira Lake with a small island in the middle. It is located about a kilometer from the Galle Face hotel. This is a beautiful park where you can walk.
These, of course, are not all interesting places in Colombo, but if you have a few hours before the plane, then visit these places.
There are no normal beaches in Colombo. We had time before departure and went to Negombo. A few kilometers from the city are beach hotels where you can visit the beach for a little money.
More photos of Colombo:
Some links:
Colombo Zoo
Sightseeing bus in Colombo
Galle Face Colonial Hotel (1864)
Getting there: The best way to reach Colombo from the airport is by taxi ($10-12). Or find a free Airport Express shuttle, which will take you to the nearest bus station, where you can take a bus to Colombo, for example, #187 (~$1).