One of our customers wanted to use a drop down choice instead of a text field to define the location where a training takes place. If you have the same requirement, this is what you can do.
The location is usually a text field and this cannot be changed. What you can do is to create your own location field and use a simple SharePoint Designer Workflow to set the SharePoint standard location field. This is important because the location is used in the meeting request that is sent to all the attendees via email when they enroll in a training.