There is a coastal road from Colombo to Negombo which, while not especially fast, is really quite scenic and worth a drive. It begins as you take the bridge out of Mattakuliya and proceed along the canal. It then continues along the coast all the way to Negombo.
This road is barely trafficked and concreted more than tarred, but it makes for a much more relaxing drive than the main Negombo Road. With the new airport highway coming up it should be barely used at all, all the better for a Sunday drive. Note that this isn’t a rush hour shortcut, however, it will still take ages like everything else at that time.
As far as long drives go, however, this strip really can’t be beat. You proceed along a canal, then beachfront, then a narrow strip of land with ocean on one side and lagoon on the other. The road passes the lovely Jetwing Lagoon and goes through to Negombo if you want refreshments, and it also connects to Colombo via the less explored Mattakuliya side. It all makes for a great day trip.