If you have the ability, in general try to have some sort of incentives for better acting, even simple things like awarding actor of the night, scare of the night etc... Simple prizes like a meal at McDonalds can help. Even without actual prizes, end of the night meetings are a great morale booster for actors. Simple awards like actor of the night give them something to work for and at the very least have bragging rights over. Or even some "premium positions" like cue line roamers are something that can be "worked for" Just a thought.
As for more ideas for easy startle scares. Pop outs of all sorts work best, as you said with drop panels, doors, classics like patterned suit against the same pattern wall, glowing mask room (with one real actor in one of the masks), etc.... You can aslo do the same ideas with lighting such as a slow duration strobe...light flickers there is no one there, light flickers again, actor is right in front of them. Another simple and effective one can be done with simple lasers or long throw disco lights and fog. Basically you can take any hallway, fill lightly with fog and project laser or other light down the hall, an actor can stand directly below the light and the customer will not see them untill they pass the light and they are right in the actors face.
You can also go the loud prop route like boom sticks, shakers, slammers, or the noise and light units that Shot In The Dark Scare Tools sells (Shock Guns I think they are called).
Hopefully those ideas can help some.
Mike "Pogo" Hach