Fun places to do science - the short version

IAP - Institut d'Astrophysique de Paris is a good place to visit - good scientists, good company, and the best little cafeteria of any astronomical institute on the planet (why, no I have not visited all of them. I stand by the statement, confident they'll rise to any challenge).
Stay either walking distance or short Metro hop away, on off-days, either get an open air baguette and vin rouge in a random corner shop, or better still by your own baguette, cheese and 1/4 bottle and have a picnic in a park. For dinner, avoid the touristy places (like most of the west bank!) and find some place good in a basement off the beaten path. Random walk-ins have high probability of being a success in this city, bad restaurants do not last long.