I take $1,000 in cash and traveler's checks for a week long cruise. But we are flying cross country so we'll probably be paying for 4 or 5 meals for 4 people at airport prices. That can easily add up to $300*. Incidentals in port can add up too. I also like to have money for an additional days meals in case we get stuck at an airport overnight.
I also make sure I have enough available credit on my credit cards to pay for one way plane tickets at last minute non-discounted full fare for all 4 of us. Bad weather, or other problem, and any sort of issue where you can't immediately get a refund for your existing tickets, or wait to file a claim with travel insurance can leave you stranded. Current last minute air fare from MCO to my home airport is $960 a person, so about $4,000.
*And debit cards aren't perfect, your card could get compromised, or lose it's data or the terminals in the airport could be down. That happened to my son at LAX and he spent 2 hungry hours there because the card readers in the entire airport were down. Dozens of restaurants, only able to take cash, and a 25 year old with no cash and an empty belly. Even $5 cash would have helped.