I don't know about S scale at shows. I know of no club in the area that I could join. However, there's a guy that brings a portable layout to a lot of shows around here, and he has a few trains that always seem to be a hit. One is a repainted Flyer train to match Lionel's girls set. Another is a Thomas set; I don't recall the details about it.
His other trains are mostly postwar. He usually has a passenger train and a freight train running. The freights have lots of gondolas and hoppers loaded with things that kids would put in there if they were playing with them (such as candy, army men, jacks, marbles...)
When I run at our modular club, I typically run old trains. Nothing scale, nothing slow. I try to run a train that is carrying something appropriate for the season or theme for the show. Our club sets up at an antique tractor show each year, so there's a lot of flatcars hauling tractors around that weekend. Christmas boxcars and the Polar Express are running during the Holiday shows. Last month I had a pair of gondolas filled with candy hearts.
Some of the stuff I run doesn't play nice with switches or crossings, so what modules are set up at a given show influences what I bring that weekend. If that's an issue with your tables, don't bring a bunch of stuff that won't run on your track. I also bring a Marx M10005 (if there's a loop with no switches) because it is so easy to pack away after the show. It's usually the last train running on the last day. This way everyone else can pack up their trains and I can pack up my other trains that take longer to pack while that train is zipping around.
Hope this helps,
J White