I miss some of the non-ride attractions as well - especially Fascination and the aquarium (I loved the fish that would just stare at the wall and never move!), and the penguins. And Jungle Larry's. But for an ampitheater, I think we might just be running into a space problem then, depending on what sort of groups you would want to book. Local groups fine, smaller, less-well known groups okay, but anything bigger may draw 20,000+ by itself, without coasters to add to the mix.
I do remember the small ampitheater that used to be down in Frontiertown and had the country show - but I never watched it, just remember seeing that it was there.
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/