What to see or do
Len 2 wrote (see)
Get on the train and go to Disneyland
Oh yes, definitely! (We went there for hubby's 40th, and would have gone for mine, but I was pregnant)
Another good way to see stuff you might not otherwise is to play Metro Roulette. Pick a station that sounds nice, and just get off and have a wander around. Great if you get the Paris Visite ticket that covers the Metro, buses and trains. If you get the Zone 1-6 version, it covers all the way out to Versaille and Eurodisney.
I did put the Disneyland across, but it wasn't well received
If you fancy something different the Arab Insitute is worth a look. Really bizarre building (one wall is built from light-reactive panels, like camera lenses) it's free to go in, and there's a cool cafe on the roof with panoramic views and mint tea.