DP & I recently spent 3 days in Paris as part of our Eurotrip back in September.

Definitely get to the Eiffel Tower at night time. The whole thing lights up and from 9pm for 10 minutes and then every hour on the hour it sparkles - very pretty! That was our only late night out.

In the mornings don't eat at the hotel, walk through the streets and go into a patisserie and buy croissants and lots of goodies for brekky.

Also recommend seeing the Pantheon and Notre Dame - the age and detail of these buildings are amazing. The Louvre is pretty impressive is well. If you are there on the first Sunday in the month it is free to get in (so line up early!).

And also definitely do a boat cruise on the Siene - you will get to go past all the tourist attractions which is good to see them if you don't get to physically visit each one.

The food is great and if you eat at a regular cafe it is not as expensive as most people think. Just be aware that smoking is still allowed in restaurants/cafes in Paris (although I think they are changing this soon?) so that was the only annoying thing about it for us.

The metro (train) is cheap and gets you everywhere reasonably quickly, but it's a bit grotty and not as nice as the train system in Spain!