This is very similar to a questin that I answered a bit ago. That answer applies here as well
Make a plan and have it on file for just such opportunities. It helps to have a pop up canopy/shade tent for these events. They aren’t very expensive, can be used indoors or outdoors, and I use them for a lot of other things. I don’t bring any heavy equipment unless there is a very good reason to do so (like a health fair that requests I do treadmill walk tests). I bring a lot of little things like a scale, a BIA, elastic tubing/bands, a few mats, fit ball, med balls, etc. I have a sheet of activities for all types of people and ages. And I will do some sort of give away drawing to get names, addresses, phone numbers and emails.
Along with that I offer to take some measurements for those interested. I have a spreadsheet that is on all of my laptops and tablets that I can put in a persons stats (weight, height, girths, skin folds, etc.) that I use at these types of events. I used to bring a printer and give them a hard copy of their stats. But now I get their email, phone, and address. Then I can use a number of different contact styles to try to get them to come in to the gym. I start with an invitation to come in and go over their numbers over the phone. If that doesn’t work I send them their stats via email and invite them to make an appointment if they would like to have the stats explained to them. The ones that didn’t bite on those two offers will get a mailer from me with a discount coupon for a service that I am promoting at the time. Everyone that gives me an address will get a thank you card for stopping by the expo/fair booth and another invite or promo offer. Occasionally I have emailed these folks again to invite them to another expo/fair coming up.
Little things add up over time and a lot of the effort gets rewarded indirectly as many times the people that I am contacting will tell others about my offers, etc.