Ultra cheap is the way to go. A 0.25€ Eiffel Tower key chain can last for years if it's hanging on a nail in the garage with certain keys, and procure pleasure every time it is seen. How long will chocolate or soap last?

Agree that the main street of Ile St Louis has a variety of nice shops where you could pick up some nicer things (soap, chocolates, olive oil, wine, cheese,etc..)

Rue de Rivoli is full of souvenir type shops, for things like T-shirts, posters, perfume,etc..

Around Place St Michel, rue de la Huchette area I bought a whole bunch of tiny gold metal Eiffel towers for gifts back home.

I've also gone to Mariage Freres (multiple stores) to bring back tea-related items.

A coworker brought back small fancy-wrapped French guest-soaps for us. That was a great gift.

If you really want something physical with the Eiffle Tower on it (frig magnets, key chains, bottle openers, etc.) you will have NO trouble finding those.

Personally I think eatable treats are the best, chocolate, a small candy or cookie or something like that with cool packaging.

I think Suze's on to something with the soaps. I bought a bunch of the olive oil soaps from Provence in the small size to put in my tiny soap container for my travel toiletries kit.

The soap containers are from Muji, got mine in London and they're also in Paris I see: http://www.muji.eu/pages/stores.asp?str=FR These little soap containers are something I searched for for years. A GREAT store for all kinds of simple (often cheap) little things.