Colombo in 2 Days: A Realistic Itinerary
Two days is the right amount of time for Colombo. One is a rush that leaves you with a checklist rather than an impression, and three starts to feel like padding unless you have a specific interest in architecture, food or shopping. This itinerary assumes two full days and works around the two things that ruin most Colombo plans: midday heat and evening traffic.
The underlying structure is the same both days. Be out early, when Pettah is trading and the light is good and it is possible to walk without suffering. Retreat somewhere shaded or indoors from roughly eleven until three, which is what everyone who lives here does. Come back out in the late afternoon and let the evening run long, because Colombo is at its best after five.
Distances are short enough that you will not spend the trip in transit, but not so short that you can walk everything. Assume a handful of tuk-tuk or PickMe rides each day between clusters, and walk within each cluster. The plan below groups sights geographically for exactly that reason.

Colombo to Anuradhapura Day Trip with Mihintale and Ancient City Stops
Bookable directly through AsiaByLocals, with a verified local operator.
From USD 62.65
Full day