must see in Paris?

I'm planning to visit Paris next Spring and wanted to find out what is a must see or do in the city besides the obvious tourist visits. I'm looking for something off the beaten path, something that isn't in a travel guide. It could be anything from a particular restaurant, a shop, an exhibit, or a type of food/drink.

I suppose this must be said by someone:

Paris is such a fantastically diverse and well visited location, that there are few
'insider secrets' to it, and those that exist, aren't given to the un-initiated, ie, those that haven't talked to people who live there IN Paris.

Its a great place, and you can spend a month seeing all the must sees that are in the travel guides before you get to the stuff that isn't published, so much is published about it.

I can say this. The catacombs are in all the guidebooks, and well worth seeing.

Last time I was in Paris, I was visiting a cousin that had been living there for about 4 years. She took us around to some great restaurants and bars, but I could never find them again without a guide.

It's a small city, and very heavily visited and well-documented. You're not going to find much of interest that isn't in a guidebook, unless it involves somewhat dangerous and certainly illegal exploration of the underground city, only small parts of which are open to public (as noted above, it is well worth a visit to those parts that are open.)

My best suggestion is to wander. Taking the metro from one attraction to another misses out on much of the city. If you walk, you won't necessarily see anything mind-blowing, but you will get more of a sense of Paris than you otherwise would. You can't get lost, as there's always a metro station nearby - you just need to know your "home" station to get back to. The actual city is not big...you can cross the whole thing in less than 2 hours on foot.

Thanks for the reply and advice. I hate travelling somewhere only to come home and find out that I missed out on an incredible opportunity or adventure.

I think the best way to see Paris is to get lost. And walk. It's unbelievable, the kind of secrets every street holds. (Try doing that at least for a day, you won't regret it)
Apart from that, Village St. Paul is absolutely precious with its little shops stuck in time and curious museums and what not. The musee de la magie on Rue St. Paul is worth a visit. And do try to check out the wine caves while you're there (they were shut when I went, but looked so inviting)
If you're visiting the Orsay, try having lunch there. It's beautiful and affordable.
Check out Da Rosa, a fun bohemian restaurant that encourages patrons of all nationalities is located inside a park - 62, Rue de Seine.
For a lovely glimpse of local life, head to Rue Cler with its fromageries, and other sweet stores and restaurants.
There's a beautiful restaurant called Chez Janou at 2, Rue Roger Verlomme, that I would most certainly recommend.
Verjus Paris does mixed tables and other sorts of interesting culinary experiences at their restaurant on 52, Rue de Richelieu, and their Bar at 47, Rue Montpensier. Do have a look.
Speak to people, including other travelers, that's how we uncovered quite a few hidden gems.
I hope you have a beautiful love affair with Paris and get to know all her deepest secrets.
Bon Voyage!

The street foods are a must-try! The'yre absolutely yummy!