I don’t know if I have any good hints, as frankly, I don’t usually hand out food on my trips. The one time I do is in the summer, when I like to stop at roadside fruit stands and buy fresh peaches or plums for everyone (and I come prepared with paper napkins and a supply of little hand wipes). And, of course, if you include a “foodie” place in your day’s tour, get samples – like chocolate factories, jelly bean places, cheese makers, wineries and the like.
Also you might buy just one thing (like a cookbook I picked up on a tour I did to Normandy) and raffle it off in the bus afterwards. I live in Northern California and we have many farms within an hour’s drive so I also like to “go green” and give city dwellers a stop at places to learn how their food is grown and appreciate our growers. A nice handout is a recipe.
Another thought is to buy something typical of the place you’re visiting on a future trip. For instance, if you were to promote a trip to New Orleans next month, why not buy some pralines for this month’s tour and hand out bite size pieces on the bus with the reminder announcement that New Orleans is where you’re going on your next trip and they should surely plan to come along with you — sort of a foodie approach to marketing.
I don’t try to compete with other tour operators as there’s always someone who can do it cheaper than you. I just try to make my trips very unique, often with the chance to meet a well-known person to whom the general public may not have access. I’m a senior myself and whle we all like “something free” we also like to come home and feel invigorated intellectually and have met or interacted with “someone important or interesting.” Don’t know if all this is of help to you, but I wish you the best!