If you offer drop in hours, I would specify what the scenario or topic will be during that time period and have some kind of sign up sheet. This keeps you from over committment and if no student signs up before a certain time, you do not have extra time and effort expended.
You can say you'll be running a CHF scenario or an OB scenario, etc. and that the sign-up deadline for Friday afternoon is Thursday afternoon. If you find out the course content schedules, it will make the activities more attractive to certain groups.Students can receive and complete any "prep sheet" activities you will have when they sign up and they will get more out of the experience. You can also have observers wait-listed so that if someone does not show up (which often happens) they will rotate into the participant role.
You may find that faculty use this as a remediation activity and require it for some students although there will naturally be some students who seek out the experiences independently.