Best approach to contacting restaurant managers?

Hi, I'm hoping members of the forum can provide some feedback on the best method to reaching out to restaurant managers in my area for a product concept I'm researching. I realize these folks are really busy and I want to maximize my chances of getting a meaningful discussion with anyone interested.
 
I'm hoping to schedule a quick and informal 15-30 minute conversation to learn more about their business and discuss a product concept I have in mind. This is purely market research and not a sales discussion of any kind. A few questions I have are:
 
- Is an in-person visit better or should I call instead?
 
- I'd like to buy them a coffee or something to thank them for their time, but given it's their establishment is there something else that would be better?
 
Any advice is welcome. Thanks!