I personally come down on the aggressive side of the aggressive/conservative line in situations like this. If you're willing to take a little risk, I'd also say the time invested is worth just a small taste of Paris, especially to someone who's never been before. The quickest "target-rich environment" you can reach is Notre Dame, as other posters have mentioned. And all-in-all, maybe that's a better use of the time. But I'm at least tempted to recommend that you instead take the RER into town and take the Metro to the Trocadero stop. Get up to street level, walk 1 block south, and take in the view of the Eiffel Tower when it opens up to the east. Then reverse the trip.

All you get is one look at the Eiffel Tower, but it unless the weather's bad, it will be an experience you won't forget. It's one of those moments when you feel like you're actually in the postcards you've seen all your life.

There's much more variety close by and much more of what Paris is all about at the Notre Dame site, and of course Notre Dame itself has a lot of the "postcard moment" factor, too, but there's little on earth like that first view of the Eiffel Tower from the Trocadero.

If this tempts you, note that you might (or might not) make better time by switching from the RER to the Metro at the Gard du Nord stop rather than going all the way to the Chatelet stop.