I think your inexperience is responsible for your opinion on spending years practicing "text book" medicine. In part this is the expected result from the way preclinical education is set up. There's a right answer and a wrong answer...while in actual clinical medicine, there's lots of "correct" answers and it's a matter of choosing which one is "more" correct (though you may never, ever know which is "most" right).
Let's start with the premise that evidence based medicine is important, because it absolutely is, but despite that emphasis, there's still lots of freedom in quite a variety of aspects of patient care. You'll see as you enter your clinical years that every attending, subspecialist, sub specialty fellow, upper level resident, and intern has a slightly different way of doing things. As you go through your training, your job is to learn the basics, and then find the approaches that you find to be the best or that your most comfortable with.
Let me give you an example in neonatalogy (because I'm a Peds intern). Anemia in premature infants is a very big concern, it can lead to increases in apneas, bradycardias and desaturation events, it can lead to feeding tolerance issues, and may contribute to lots of other afflictions. While there are physiologic reasons for anemia in preterm newborns, there is also a significant iatrogenic component as we physicians push for frequent blood gasses, worry about electrolyte imbalances and try to manage infections and necrotizing enterocolitis. With all that going on, a great many preemies will receive blood transfusions. But there are LOTS of different ways that people go about giving transfusions - some will run 10ml/kg in over 4 hours, hold feeds and give lasix to avoid volume overload, others will do two 7.5ml/kg transfusions each run over 3 hours spaced 6 hours apart continue regular feedings and not give lasix. And you can easily come up with lots of variations on these approaches...Why the differences? Certainly the patient's clinical status matters, as well as what each doctor perceives to be the risk of things like hyperviscosity of the blood and what that may impact in later management (ie, the feeding/not feeding issue - some believe that with an increased viscosity of blood you'll get hypoperfusion to the gut and feeding a hypoperfused GI tract is thought to be a risk factor for development of NEC which is of course very serious). Experience also plays a major role in making the decisions.
Now perhaps this isn't the type of creativity youre looking for. And, there's always the chance that a study will be done to answer the question "what's the best way to do X?" in which case you would be doing your patients a disservice by not following what the research has identified with superior outcomes. But the point is that there is still a lot of "art" when you get down to the details of managing patient care. If there wasn't, by now we'd have a computer that could determine the next step in treatment, write the orders for the nurses, and follow up on the results, with none of the hassles that come from having physicians work 30 hour (or longer in the cases of attendings) shifts.
So really, all fields of medicine are going to allow you to have freedom and creativity within reasonable limits to treat your patients. Some fields though are obviously going to have more than others:
Someone mentioned Psychiatry - certainly true if you are into doing psychotherapy, but many psychiatrists are largely in the model of managing medications because they get paid more to do that, leaving the counseling to the clinical psychologists instead.
Some others that stick out in terms of "creativity" to me: Anything surgical because there's really no instruction manual when it comes to the OR - there are steps and things that can't be skipped over, but the path one takes to get to the final outcome is up to the individual surgeon.
PM&R - when you're designing physical therapy plans, there's lots of leeway in choosing different modalities.
Pediatrics - because there's less data/studies done with kids to provide evidence based medicine guidelines - especially in subspecialty pediatrics. However, there is a lot of 'well this is the way we've always done it' in peds.
