As far as street food/kades in Colombo go, the Yarl Eat House is one of the best. They do a good rice and curry with your choice of mutton, prawn, cuttlefish, crab and other delicacies, all on a banana leaf.
The basic Yarl meal (for lunch) is rice on a banana leaf with three veg sides that they ladle onto your plate (as it is). These are three veg, usually purippu, a sort of sambar and something else. If you're vegetarian this is a pretty good meal, though you'll have to ask for a bit more than they usually give.
On top of this they also give you a metal bowl with your choice of meat sides. The general options are chicken, mutton, prawn, cuttlefish and crab. Half a fish is included without asking. The chicken is a bit bony and unrewarding in our experience, but the other sides are excellent. Yarl is especially famous for its crab which is generous, though it brings your price of meal up a bit. I like the mutton very much, tender and spicy. Note that everything is quite hot.
And that's basically it. If you have a few meat sides the price of your meal can go up over Rs. 400, but you can eat quite happily for 200 Rupees a head. The rice and curries are prepared in a more Tamil style (the name Yarl refers to Jaffna) and it's quite flavorful and good. If you go here a few times you start to develop a craving for it.
The Yarl is quite literally a hole in the wall. As you go down W.A. Silva Mawatha, past the Galle Road, it turns into Station Road. Not five meters in, you'll see this tiny restaurant. There is no parking at all down the narrow lane and it's recommended that you take a trishaw.
The interior is quite simple, just metal tables and chairs and a hole in the wall to put your banana leaf through when you're done.
The service here is generally excellent. The same guy has been working here for years and he's quite happy to recommend stuff. You get your order fast but you're expected to clear out as well, it's a busy spot for lunch.
The Yarl Eat House is perhaps Sri Lanka's best street-diner, serving spicy crab, prawns, mutton and cuttlefish along with a solid rice and curry, all for a reasonable price.