Taco Bell is a nice place to eat and the food is good for you in my opinion, I find that most of the garnish they add to it is fresh They use a lot of vegetable prepared meats and cheese to make their tacos, that eliminates a lot of grease and fats, I could eat it every day and not gain any weight.
This is not a strange topic. More and more fast food restaurants are trying to offer healthy selections. It seems like almost everyone including McDonnald's is offering a variety of salads. Be careful about the salad dressing, because it tastes good, but no longer healthy. However, fast food is generally the epitome of unhealthy, I think your best option to go somewhere healthy is something like Subway.
I don't think that there really is. Fast food is a fast food. They are supposed not only to serve us fast, but also to be cheap. It can't be fast and then healthy and cheap. If it were fast and cheap, it must not be healthy. If it were fast and healthy, expect a skyrocketing price, something that would have you cash out a high amount of money, price that's only affordable for the rich ones.
I know that everyone else on this forum is saying Subway, and I actually agree. It isn't completely healthy though, due to so much sauce and dressing they normally put on, but you can address that to them by asking them to lessen it if you'd like. Subway promotes it's sandwiches as fresh, healthy, and all natural. Hopefully they sell their sandwiches the way they preach them to be. Other than that, I love Subway and that fact that they have pretty fast service. It's good for people who are on the go.