Before heading back to Paris for the fourth time, I looked up "under the radar" types of activities and came up with a decent list. Unfortunately, a few of these were weather-dependent (like hopping in a giant tethered hot air balloon to get cool views of the city) so I had to cross a bunch off as infeasible. But, we did manage to hit up Coulée Vert, also known as Promenade Plantée, which was described by a few people as Paris' version of The Highline.
You can certainly tell that yes, this was once a railroad. However, it's a little bit opposite of The Highline in that you walk downstairs instead of upstairs to access it.
Okay, so here's my pro tip for those of you who want to see the Eiffel Tower but don't necessarily want to deal with the crowds. Take Metro line 6 (the light green one), preferably from Sèvres-Lecourbe station heading in the direction of L'Étoile. Sit on the right side of the train next to a window - hopefully one that's relatively free of scratches. You'll see glimpses of the tower in between buildings as you're traveling along. When you're in between the Bir-Hakeim and Passy stations, you'll get a really good view of it with the Seine in the foreground.
Montparnasse Tower. You'll see some awesome views with the Eiffel Tower in your photos which I think will be prettier to show off to your friends, right?