I spent Christmas week in Paris in 2012.
Get advanced tickets through the internet to the Eiffel Tower otherwise you will stand in line for hours. It might be too late to get advanced tickets to the top, but you might be able to get advanced tickets to the halfway point. If you're determined to go to the top, my guess is you'll have a three- to four-hour wait. There is intense security around the Eiffel now.
I strongly recommend the Catacombs for your sons. Try to get advanced tickets through the internet to avoid lines.
Maybe your sons would be awed by Sainte-Chapelle, the church only 2 blocks from Notre Dame, with the dazzling stainless glass. It's not just another church. Everyone is awes by Sainte-Chapelle.
They also might enjoy a visit to Sacre Coeur in part because it's on a hill overlooking Paris. They could walk off a lot of energy.
Maybe they would like to attend a can-can performance at the Moulin Rouge. It's not far from Sacre Coeur.
Maybe they'd like a river cruise on the Seine, especially at night.
The Hop-On Hop-Off tour is made for teens (if they are teens).
And as you mention, a bicycle tour and Versailles tour would be great.