I think both the Erawan Museum and the Ancient City were created by the same person/family/group?

There may be taxis waiting inside the Ancient City, or maybe they can call for one, or dropping people off? I drove my own car, which we also took inside the park, which makes it very easy to get around vs. walking, biking; they also have golf carts.

Maybe use the GrabTaxi/Car, Uber apps? Or at least have them on your phone.