1) Do not take the RED tour bus (or ANY tour bus for that matter). They are absolutely horrible and a scandalous rip off. They have english audio spoken by a french woman with an obvious french accent making it hard to understand anything, literally next to no information (it only speaks to say the names of the monuments and maaaaybe one fact about them) and the rest of the time it plays tinny horrible quality classic music on loop. It was the worst bus tour ever - and I enjoy doing these things in general.
2) Eiffel tower, it's going to be a long wait but worth it..
3) Catacombs, I tried going but the queue was more than 300 people long, which meant a couple of hours of waiting, so I left..
4) Louvre, is definitely worth a visit but you need a whole day for it
5) Versailles, worth a visit of course
6) Notre Damme, very nice and worth a visit
7) Walk around Champs Elysse, grand palace and to the Eiffel tower, very nice walk, worth it
8) Eurostar to London: priceless..
Seriously, Parisians are rude, unbelievably poor in hygiene (get on their tube and you'll see what I mean) and their customer service is infamously horrible. Prepare to be completely ignored if you don't speak french and for some very dirty waiters and waitresses (handling your glasses and bottles from the rim and the top with open palms etc..).
It's worth a visit, Paris, but not much more..