Oh yeah, I almost forgot, try to use $50 and $100 bills. You'll get more stares and comments than you would think. A $100 bill today is worth less than $20s in the 1960s, but no one in the 1960s commented when you used a $20 bill.
I my part of the world, you need to keep significant cash for when the hurricanes take down the power grid. You would be surprised by the number of small stores that will open up even when they don't have electricity, because they get customers that they otherwise would not get.
Every piece of paper currency reads “This note is legal tender for all debts public and private.”
Technically, if an establishment refuses to take the cash proffered by you, they should give it to you for free.