- as an adult and a web developer: https://github.com/abelards/how_tos/blob/master/paris.md
- with kids, from @jlecour and friends: https://gist.github.com/jlecour/6088638
- as a young adult/teen: see below
As locals we like parks & gardens but you didn't come to Paris for that? ;)
- Eiffel Tower
- Versailles (expect 1h in public transportation from Paris' center)
- Disneyland and the smaller but less crowded Parc Astérix
- History : Notre Dame, Crypte Archéologique, Basilique de Saint Denis (dead kings + some treasure)
- Science : Muséum d'Histoire Naturelle, Cité des Sciences, Palais de la Découverte, Arts et Métiers
- Art : Georges Pompidou, Cité de la Mode, Musée d'Orsay...
- Maze/escape games : seriously, just laugh at CDG airport and Chatelet/Les Halles. We're sorry.
- World Cultures : Orsay, Guimet (Asia), Quai Branly (Africa, Asia, Oceania, Americas...)
I'm trying to think of things you can do here that you really can't elsewhere
- walking along the Seine or any Canal, on the Coulée Verte (near Bastille, it's like NYC's High Line)
- passages (couverts) little (roofed) streets with curiosities, bars, restaurants, art dealers...
- bateaux mouches (many boat companies offer a short cruise on the river Seine: you'll see the monuments but walking on the banks is cool too)
- égouts de Paris (how sewers work, including a real stretch of currently used sewers)
- catacombes (skulls everywhere, can be quite cold, a big queue to get in, and there are many stairs/steps so no disabled access)
- the Rocky Horror Picture Show at Studio Galande (one of the few places it's still shown twice a week -- book a week in advance)
Many areas have secluded spots just beyond the crowded streets, each with their culture. e.g. I like la petite Alsace (very cute small houses area). If you have a taste for history and architecture you'll know what was for the poor or for which immigration wave. https://duckduckgo.com/?q=petite+Alsace%2C+Paris&t=fpas&iax=images&ia=images