If you play some of the more popular community modules in NwN Hall of Fame, Tortured Hearts I, by SubBassman has day cycles, NPC nocturnal and diurnal behaviours, realistic gold weight (2000gp = 1 pound, iirc), and a bank for storing excess cash (fee applicable), and diverse and disparate shopping, to name a few.
Rick Burton incorporates city rules, such as no shields and unsheathed weapons, in Twilight, with an automatic macro to holster weapons and shields upon entering a city, and vice versa.
Of the two, I found realistic sleep patterns difficult to manage: fatigue seemed to be difficult to manage with the PC and party, so even when I wanted to sleep regularly, it always seemed to get out of synch and just be a pain. (There was no option to "sleep in", for example, so if you went to bed at 6pm you would wake at 2am!)
Adam Miller's Dreamcatcher 3 started off in an under-city dungeon diseased and weak, with no weapons, armour, health, food nor light source: a brilliant idea (and one that my Monk class PC excelled at. )
Multiple currencies is more trouble than it's worth. I'm all for adding weight to gp; it's kinda silly that a munchkin can kill an ancient red dragon and then scoop up all the ph47 1007 and walk back to town, without a hernia or an angry rogue in sight ...