most local tour companies should be able to arrange a car/driver for you. That's one of the main ways people see the island. I did a 17 day trip around the island with a driver/guide.

Trip Advisor should have some tour company reviews, I think.

For the person with only 24 hours, I'd say you would barely be scratching the surface. Colombo isn't really the greatest city....lots of traffic jams and not much in the way of sights anyway. I don't know where you're going to or from but the idea of just spending the time in Negombo does make some sense if you just want a beach. Given how long it takes to get from place to place I wouldn't probably go too far afield. But if you are really ambitious I suppose you could get to Dambulla or Anuradhapura. Google maps seems to think you can get there in about 3 hours but I recall it taking much longer. I suppose another consideration would be exactly when those 24 hours start/end...