It is very important to do whatever you like. Research experience always seperates applicants into 2 groups; and I'll let you guess which group is preferred by adcoms. Despite this, it is NOT NECESSARY to do research. If you're not interested in MD/PhD programs then I'll have to say that it doesn't matter what you do research in; anthropology, sociology, digital logic, human $exuality, history, philosophy, mathematics, balh blah blah... It's all good. All that I would consider necessary is that your research be RIGOROUS. How rigorous? Well, enough to present some poster at the regional or national society of historians, anthropologists, mathematicians, yada da. What I'm saying is that it's not enough to just read a couple of papers in the area. You MUST have a mentor in the department and work closely and hard with this person for a year or more.
To find this mentor person, just decide the area you want to do research in; then go to the department or field website and pull up names of people whose work you find interesting. Narrow your list down and then read what they do and what they have recently published. Next, just send emails to each of them introducing yourself and asking for an appointment to come and speak with them about their work which you find EXTERMELY fascinating! They'll usually all respond positively. It's that easy.
If you're interested in MD/PhD programs, then you should do wet-bench biomedical research.