It's been a while since I applied but in checking out their website now, it looks like a very simple process. Have a look at the BMO Harris Personal Checking Account section at:
https://www.bmo.com/main/personal/bank-accounts/cross-border-banking#. You can info and at least begin the process of applying for an account online and they have options for Canadian residents and citizens. Being a bank, you'll have to send them a copy of some personal ID to set things up. Unfortunately I don't remember what it was for me (again, years ago...).
Their BMO Harris Smart Advantage checking account has no monthly fee if you opt for electronic statements, and they'll send you both a BMO Mastercard debit card and a starter kit with some cheques for your U.S.-based bank account.
BMO Harris branches are mostly found around Chicago (super handy if you're flying to or connecting through O'Hare) but there are a few ATMs, if not branches, in Florida as well I believe. Also, they're part of some ATM alliance that gives you free withdrawals in 40,000 ATMs all over the place so it should be pretty easy to get at your funds.
Seems you can transfer US$ from a Canadian BMO US$ account to a BMO Harris account but not the other way around. There are other ways to fund the account however.
Finally, if you have a U.S. mailing address, you're golden. If you don't have a U.S. mailing address, you might run into some problems buying stuff online from U.S. sites since even though you'd have a U.S. account, the checkout process on some sites requires a U.S. address. Sometimes you can shoehorn a Canadian address in there, sometimes not.