On the booking page there should be a sub category option. An example for us is we run a travel clinic and need to know what vaccine a client wants before they come in so we can make sure we have stock of this. Example;
Patient selects travel clinic (30 mins) which then has a drop down menu for them to select vaccines that we have pre populated
This way the appointment time is not impacted but the patient knows which vaccine we offer and also we know which vaccines the patient wants before they even arrive