Have a great trip! I went to Paris alone for a week over the summer and had a fabulous time. I really kept myself busy - went to tons of museums and walking around and it sounds like you've identified some places you want to go. Don't discount the idea of just wandering around and giving yourself plenty of time to check out a place you've just stumbled across.

I pretty much kept to myself the entire trip, but making yourself a regular at a cafe near your hotel sounds like a good idea.

As for the Marais, many places observe Shabbat, so it's very quiet from sundown Friday 'til sundown Saturday. I really enjoyed the falafel at L'As du Falafel. You can either sit in the restaurant or get a falafel to go from the window out front.

As someone else pointed out, get familiar with the metro as well as the bus. I found the bus to be a great way to get around. The maps in the bus shelters were easy to understand.

Enjoy!