X-com/X-com:Terror from the Deep if you can run them on XP, with the edit-everything editors.
Jagged Alliance 2 with the v1.13 mod.
Sid Meyer's Pirates! 2 whatever it's called, or Pirates of the Caribbean with the major improvement mod which turns the game into Pirates! with a small map.
Mount&Blade, not much gameplay yet because it's still in Beta.
Rome: Total War with the mod of your choice, but never vanilla, I suggest EB or wait for RTR7.
Fable: The Lost Chapters, it's plenty scripted, the world is tiny, and you will finish it very quickly if you don't stop, take it nice and slow and enjoy yourself, but there are freedoms here and there, and like KOTOR, there's always a choice between Good and Evil.
X3 you already tried. HoI-II was already suggested, for a "son's game" that can get you addicted, you can try Worms 4, though it's a multiplayer game, you need to play with "the son" or convince him to let you play with him and his friends.
Did you ever played F4? If not, go buy and play F4:AF, or any other installment of F4, just to check the campaign. It has the best campaign engine by all standards I have ever tried, in many years. It's worth it just to see it if you never touched it before, though be warned, the experience will dumb down and minimize every other campaign system out there.
We are lacking games of this kind, too many gamers simply want a quick fun session instead of diving deep into a game, gladly, I hope Mount&Blade will prove a game can be deep and freeform and still entertain the quick action gang. I suppose if Worms 4 had a strategic map it would suit them too for massive sales and potentially add some sort of freeform RTS game.
Keep your eyes on the Silent Storm franchise, it is said the next version will feature and open-ended campaign map much like that of Jagged Alliance 2 in concept.