Hard for me to make any suggestion of serious reference. I think it's going to come down to the market demographic you are selling in, and that can vary wildy within the same general location.
What is popular at one market may not be selling well at a market 10 miles away. Of course even though it may not be selling on that weekend doesn't mean that it won't be selling next weekend neither.
If I had to put my money on a product that sold just about everywhere, I would be backing coffee/food. That's about all I would be confident about if I was doing a selling world tour of markets.
You know these types of markets also catter for niche products, so do some research on your local market. See what booths do get the most traffic over a month or so, then see if you can hone in on developing a niche to cater for what they are already buying.
If plants are big sellers, see if you can sell plant food, pots, that type of thing, and so on. One product will usually lead to another product, and finding a niche market is all about looking for a link in that product chain that is not being catered for.
Just be careful not to try and research seasonal items, because those types of market products are always going to be popular within season, but may well suck after season.
For example, Christmas. Of course market stall related to to the season will be popular within that season, but unless you are wanting a short gain, including them in your research is only going to give you unrelaible data for a long term strategy.