CO's Freedom Flight Club gives a 20% across the board discount for weekday travel and there are no extra restrictions. for example, if you buy a weekday Y at 20% off, it is marked Y, and you have all the perks that come with full-fare.
Even though the international discounted fares are upgradeable with miles, you need to deal with CO's difficult policies in that
area, and unless you are Elite, there may be no uograding opportunities.
Additionally, I do not know if the FFC is available ex-US.