Most travellers flying into Sri Lanka land at Bandaranaike International Airport, which sits north of Colombo. A large number of them spend a night or two in the city before realising they could have been on the south coast already. Here is an honest take on how long Colombo deserves and how to move on from it.
How long to spend in Colombo. One day is enough. Colombo has a handful of things genuinely worth seeing and a lot of things that look interesting on paper but are not. It is a working city, not a tourist destination, and it rewards people who approach it that way. Spending two or three days there when you have limited time in Sri Lanka is usually something travellers regret.
What is actually worth your time. Pettah market is the most interesting part of the city, dense, chaotic, genuinely local, and good for cheap food if you eat where the workers eat rather than at tourist spots. Galle Face Green in the early evening is worth an hour, locals come out for the sea breeze, vendors set up along the promenade, and it has an easy, social atmosphere. The Fort and Beira Lake area is pleasant for a walk. The National Museum is worth visiting if you are combining with Kandy or Sigiriya and want historical context, less so if you are here only for the coast.
What is not worth your time. Most of the shopping malls. The overhyped rooftop bars. Guided city tours that take you to fixed-price craft shops. If someone in a tuk-tuk offers to show you Colombo's hidden gems, they are taking you to a shop.
Accommodation in Colombo. It is expensive relative to the rest of Sri Lanka. A decent mid-range hotel in Colombo costs what you would pay for a boutique guesthouse with sea views in Weligama or Galle. Unless you have a flight connection or a genuine reason to stay, use Colombo as a half day stop rather than an overnight.
Getting from the airport into Colombo. The airport is in Katunayake, about 30km north of the city centre. By taxi it is 45 minutes to an hour and a half depending on traffic. The expressway cuts the time down significantly but taxis that use it charge extra for the toll. Agree on the route and price before getting in. The airport train is a cheaper option and takes about 40 minutes to Colombo Fort station.
Getting from Colombo to the south coast. The coastal train from Colombo Fort south toward Galle runs several times a day. It takes about 2 to 2.5 hours to Galle and the section running along the coast is one of the most scenic stretches of track in the country. From Galle you can take a local bus or tuk-tuk onward to Unawatuna, Weligama, Mirissa or further east. A second class ticket from Colombo to Galle costs around LKR 100 to 200. A private driver from Colombo airport direct to the south coast costs USD 40 to 55 and takes 2 to 2.5 hours outside of traffic. For two or more people this is often the cleanest option.
On your way back. If your flight home is early morning, consider spending your last night in Negombo rather than coming from the south coast on the day of departure. Negombo is five minutes from the airport. It is not a destination worth a special trip but it solves the logistics problem simply and means you are not rushing a 2.5 hour drive on four hours sleep before a flight.