Kaweh Cafe is one of the many options that you will find in Nimmanahaeminda area and probably one of the best.
The place is quiet and very spacious with all sorts of tables and seating; big tables for group projects, single ones and others a bit more isolated if you need more privacy or to Skype. Most of the spots have plugs so you don’t need to worry about your laptop being charged before you leave.
The coffee is good and they also serve some very basic sandwiches if you don’t want to leave for lunch. Internet is very fast but it can disconnect from time to time — at least that happened when I was there. Overall it’s a mostly perfect working spot.