I don't have FS2K, but I do have the Concorde SST add-on for FS98, which is very nice.
I like doing the actual routes flown by Concorde (I know, not very original am I!). These include the inaugural London-Bahrain route, CDG - JFK, LHR - IAD, and the route that already comes programmed with the add-on, LHR to JFK and back.
For the LHR-JFK, CDG-JFK, and London-Bahrain routes, I've obtained the actual waypoints of the routes flown by Air France and British Airways (Tracks Sierra Mike and Sierra November for the North Atlantic routes. I believe there is one more but I don't have the info).
I don't know how good the flight model for the FS2K Concorde is, but this FS98 add-on is superb, and it's really fun trying to accurately follow the flight plan used in the real world. This add-on also comes with a nice inertial navigation system, which lets you program up to 26 waypoints. One neat feature are the groundspeed and time to next waypoint buttons. The first time I flew it I stared in amazement watching the distance to next waypont indicator decrease very quickly!
If any you are interested, I have the waypoint set for the CDG-JFK route in excel spreadsheet form. I'm going to type up the LHR - JFK, LHR - IAD, and LHR - Bahrain routes as well, and I'm trying to find info on the route flown from Bahrain to Singapore as well.