I usually only travel w/ $$ for tips, and snacks. We hate carrying cash at home, or on vacation.
If you are staying on site, you can use your room key, as a charge at most places. A few snack carts are realy the only places that don't accept them, and a few spots in DD. You just leave a credit/deit card number when you chek in. Much easier that hauling around large sums of cash. You can check w/ the front desk from time to time to check the balance, and make sure you are not overspending.
As for amounts. I like to plan about $200/day, for our family of 4 (2 adults, 2 kids), for meals, and soveniers. We usually do not spend that much, but a few days spend more, if we have a more expensive meal.