As I think about the concept of a vacation theme, I think the boys have a point. It's not much different, really, than a tour of aquariums or a road trip involving visits to different major league baseball parks. It's a thread that links otherwise different places.
A scavenger hunt
While finding a common theme suitable for a trip can be tricky, keeping to it can be a little like a scavenger hunt - and that can make it a lot of fun. For example, beef jerky might seem fairly obscure, but it isn't really. Every time we've gone on a trip and gone into a convenience store, there has been a local beef jerky brand - some better than others, of course. If the local brand wasn't entirely obvious, we considered it a personal challenge to locate it. It (hopefully) will be the same way on this trip.
To make a theme work for you, let your imagination fly! Think about those things that make your family unique, and favorite types of places your kids like to visit. I once heard of a man on a mission to visit every single Starbucks. While such a level caffienation might not be - well, isn't - appropriate for toddlers, perhaps a searching out regional sodas might be fun. Or minor league ball parks, or unique local museums, or the biscuits with sausage gravy at local independent diners, or just about anything you can think of.
A geeky kind of fun
To make this beef jerky theme really complete, the boys have come up with a rating system and written it down on index cards. For every brand we try, everyone in the family will rate overall taste, saltiness, spiciness, overal texture, relative toughness versus tenderness, and so on. When we get home, we'll tally scores for each brand and declare a winner. And what are the benefits of "winning" for the highest rated brand? Nothing really, except a note in a family album. I sure hope that is enough.
Then it's up to their geek parents to apply some light statistical analysis to the results. Are the better brands clustered in one area? Is there an ingredient in a favored brand, or the lack of an ingredient? There are any number of ways to record and/or evaluate your theme, from the super-geeky like I've described, to the simple, like a book of photos recording your theme.
For us, this theme is a bit of sub-text to our mail goal of being together and getting away. It's not the most important thing about the trip, but it's an awful lot of fun.