h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Definitively, the best view of Paris is from "la tour montparnasse" since it's the only place in paris where you don't see "la tour montparnasse" which is said to be one of the uggliest building in Paris
my favourite place in paris are "les bouquinistes", those are people selling books and typical parisian images (like the one of the black cat "le chat noir") in the street in some sort of green boxes
I'm not really sure about wether other people like it but it's really pleasant to walk there when it's sunny (and you could go to "paris plage" paris beach if it's in summer)and it's typically parisian
the thing about the underground is totally right : in the underground, just forget any rules of politeness you might have learnt (for intance don't ask people if they can move a little bit if they are just in front of the exit and want to go out the underground : just push them and bump into them) : they'll do the same
Whatever happens, you shouldn't ask your way especially your accents betrays you're a tourist : most people are tourists as well and don't have any idea, a large part of the rest don't know either but will indicate you the first way they'll see (the police do it as well so beware), the other part just don't care about the fact that you're lost and won't even stop to tell you so
hope this will help a bit
Chupito
PS: mona lisa is "la joconde" not la jaconde

I am off to Paris for a weekend break this weekend.

What are the best things to see and do in the 3 days I am there?

I hope this answer won't be too late,
I thing you should do the usual things : the Louvres, The Effel tower, the Seine's bank and bouquinist, a tour on "bateau-mouche" (fly boat), the Sacre coeur ...
you should find them quite easily, all of them are well indicated