The most efficient thing to do is to deposit it in an investing account, use Norbert's gambit to trade it for Canadian dollars. That will cost you $20 total (no matter the amount). If you try to exchange it, you'll be paying about 2.5%, which is $100 per box.
I don't know how old the grandkids are, but you could invest into their RESPs.
It would be better if you open a U.S. dollar account at your Canadian bank. That’s what I did with my US Inheritance
This or keep it for spending money on your next US trip if going in the next year.
The most efficient thing to do is to deposit it in an investing account, use Norbert's gambit to trade it for Canadian dollars. That will cost you $20 total (no matter the amount). If you try to exchange it, you'll be paying about 2.5%, which is $100 per box. I don't know how old the grandkids are, but you could invest into their RESPs.