usually I'll use them to buy NMT or the Nook Miles furniture. since I'm always letting villagers leave I never just have a large number of nook miles sitting around
i have 225 000 or something like that and i don't ever go out of my way to complete tasks, i just redeem my miles when i happen to clear a task. i barely do anything with my miles at this point tbh, i have enough nook miles tickets to last me for now so i don't feel like clicking through the whole printing process over a hundred times to redeem all my miles for tickets
I'm saving them for now since I have nothing to spend them on. I'm waiting for Nintendo to release more nook mile items we can get from the ABD, but who knows if that will happen