-Yard tracks with switches to turn power on and off for setting up trains without disturbing other operations.
-Trolleys or street cars always seem to be popular (at least around here), especially with those who remember riding them when they were more common in the U.S.
-I always like seeing scenes, buildings or other things related to the town the layout is in. In my annual Christmas display at the Redford Theater, I try to work in touches of Detroit to the town, though the town itself isn't modeling Detroit. The Glancy layout also has some buildings set up to look like former or current Detroit landmarks.
-Bridges are cool
-Because my Christmas display is in a pretty dark area when the regular house lights are on at the theater, I like using plenty of lights so the layout can light itself up.
-Small scenes or other things people can look for. I have a bunch of Santa Clause figures that I distribute that people always like looking for.
-Change things occasionally. We have several lighted billboards that we put advertisements for our upcoming movies in. As movies are shown, we swap out signs for the next one.
-As stated, O-72 curves and up is always a nice thing (maybe a minimum of O-60). Even at O-60, you can run just about anything on the layout, but O-72 would be even better.
-The Glancy layout has a Lionel Transfer Table on it which is always a crowd pleaser when we operate it. I usually bring a couple smaller switcher type locomotives to demonstrate on it.
-If you're having kids visit (or ever plan to), push buttons around the fascia board to operate a few accessories are fun. Even adults like pushing the buttons.
Good luck with the layout.