The aliens just want to observe as many and as varied people as possible, no probing necessary!
I didn’t think about the lure of food safety, brand familiarity, and packaging. It is fair to say that the aliens have the ability to mimic name brands and their packaging if they need to.
I would go with pizza. (I assume they can do different toppings/crusts at these food trucks?)
Coca-Cola is also a good answer, but I’m hungry and I want pizza. 
My initial thought was chocolate, but after having read the answers I really like this idea. Chocolate is something most people would enjoy partaking of if they’ve already wandered over, but to get people to change directions and wander over to your feeder in the first place? Can’t do much better than glazed cinnamon rolls!
Ummmm! Random Street Sushi! 
Bacon. The smell of it cooking would attract millions.
Chips. And that’s actual potato chips to the Brits among us. 
Fresh roasted nuts, local varieties if at all possible. The delightful smell will get attention, and the big “FREE - fresh roasted local nuts!” sign will bring them over.
This was going to be my answer. Some of the suggestions upthread would actually repel me. 
Oooooo. I saw this one after I posted.
The fresh roasted macadamia nuts at the Mauna Loa factory definitely attracted me. And I’m so glad I bought a bag. I was with a very frugal friend who kept trying to get me to save money by buying canned Mauna Loa macadamia nuts at Hilo Hattie’s.
I went with her suggestion for taking some home. Big mistake. Cheap, yes, Stale, also.
Well, you can’t eat just one.
Butter
Using psychology, I think the narrative around the food would be very important too.
You can’t just give food away, people assume something is up (unless it’s a corporate event) . If you sell it at market value, people will bypass it.
If you have it 80% below market value and claim “the factory made too much, eat it before we have to throw it away” a lot more people will but and eat it.
I don’t know if that’s relevant to the OP, but there you go.
For a quick snack I think people prefer sweets over savory foods.
Milk chocolate.
Beer, it doesn’t even have to be good beer. How good does free beer have to be?
Popcorn. That’s what movie theatres always used.
I do like the ‘coke’ idea too though
Fried clams.
To hell with the aliens’ diversity program, I want my clams.
I have an M&Ms dispenser on my desk and I think everyone on my floor has come by to check it out at one time or another. I have peanut butter ones in there most of the time because I like them best. The dark chocolate ones are popular too.
And the tag they attach to your right earlobe hardly hurts at all after a little while.
Marijuana brownies, the aliens could get by with a lot of probing without complaints.
Might very well incite billions.
Twinkies!