I have been to the lighting of the tree several times and from my memories it is pretty much orientated towards kids. Be sure to get there early because crowds are pretty big depending on weather of course. But again from memories I don't think they do much more than have a few kids sing then light the tree. It is kinda similar to New Years Eve.
I have lived in Indy my entire life and have never ate at St. Elmos but it has an excellent reputation. If you want reservations then I would suggest calling ASAP. All the other restaurant selections are good. Around Circle Center mall there are many other chain type restaurants that are always good. Some non chain restaurants I would recommend if you like Italian are Iaria's or the Milano Inn. Both are downtown and have excellent food.
For nightlife check out the Rathskeller. It's a pretty cool old style german restaurant pub type place that is always a good time. http://www.rathskeller.com/