I generally do both -- carry around $50-100 for expenses or much more when traveling, but put things bought online (plane tickets) or which require deposits (car rental/hotel) on the credit card. Though I've paid cash for hotel rooms: after taking a small deposit, they were fine with it. Come to think of it, I've paid cash for a plane ticket in Europe with no issue as well (Krakow to Warsaw shuttle, went to the airport and bought the ticket like it was a train )

A lot of places in NY are generally reluctant to accept plastic or have minimums, so cash is always good to have around.

PS - your brother sounds like an awesome guy. If more Americans were like him, this country wouldn't be in the toilet that it's been in for the past 10 years.