The signs start literally along the Kentucky State Line and badger you to SEE RUBY FALLS. It's a waterfall in a cave and it's alright, but hardly the epitome of Chattanooga. And you'll PAY to see it. Most of the other attractions are, in my opinion, superior to this, and FREE!!!!
Unique Suggestions: If you must "SEE RUBY FALLS", do it on a hot summer night and let the spray cool you down in the cave area.
Fun Alternatives: Spend some time wandering those mountain roads, admire the houses and views. Check out a Night Drive and watch the planes come in from a distance to Lovell Field. The lights from the mountain at night are amazing. You probably don't see views like that except for Denver, Pittsburgh and a few other major cities.
The Choo-Choo is the worst tourist trap in the world. Really. It's awful. It's just a Holiday Inn made of railway cars. Don't bother -- you'll just upset yourself.