To learn the business, you run the first cart or truck yourself and then, because just one of them is unlikely to generate sufficient income, once you've got one running well, you keep cloning it until you've made as much money as you need. To minimize employee theft, hire your friends or empty nesters to run the subsequent carts/trucks, and treat them well.
The new variation? Gourmet food. How about sandwiches like organic turkey sandwiches with pesto and Danish havarti cheese and heirloom tomatoes on ciabatta bread? Or French crepes--the savories with imported ham and gruyere, the sweet with gourmet chocolate and whipped cream? Or baby lettuce salads with fresh pears, gorgonzola, and candied walnuts?
You can charge $2 to $3 more per item for food that will cost you just pennies more--and you'll be a cut above other fast-food operations, mobile or store-front.