Stay in Negombo instead. Colombo is an extremely busy city, with a beautiful water front and a couple of good restaurants, but Negombo is where the natural beauty is at. And it’s way closer to the airport.
Call Maria Boat Tours. (Trust me, it’s no luxury yacht but is totally WORTH IT!)
Hop over to Colombo for a day. We used Uber and it was way cheaper than the 3000 Sri Lankan rupees that a taxi guy quoted us. I believe we landed up paying about LKR3000 in total for the entire round trip. Uber is safe and reliable though there aren’t many available in Negombo.
In Columbo, visit the Buddhist Temples, Taj Samudra, the Ministry of Crab, Gallery Cafe, and more. You don’t need more than a day for this.