And here's another idea,The Agora Saint Germain http://www.hotelagorasaintgermain.com/index.html
I have stayed here with a friend on a girls' only trip but would feel comfortable staying there on my own.
The hotel is an easy walk to the River, and to Notre Dame ,and close to a Metro stop. There are near-by restaurants and a street market.
The front desk personal were friendly and helpful.

We stayed in the Pantheon in March and it was good


http://www.hoteldupantheon.com/booking.html

They seem to have vacancies, but I don't know your exact dates or budget. There re links to other hotels in their "chain"- all in the Latin Quarter.

It'll be that RWC again.

We stayed at the Hotel Bucci, it is pricey, but a great location for walking, shopping, restaurants etc. We walked to Notre Dame and the Louvre, although it is across the river. We like to walk everywhere, St. Sulpice is within walking distance too. Wonderful art deco int with lower level bar and internet area. Great staff.

Yes, the Hotel Buci is in a great area. That's my neighborhood of many years. I remember when that hotel, before the renovation and possible name change, was around $18. a night. Smiles. A few steps around the corner on the rue de Seine, you can also try Hotel de Seine at 52 rue de Seine. I stayed there for 9 years and then there's the Welcome Hotel another few steps away on rue de Seine owned by the same owners as the Hotel de Seine. I also stayed there for many years. Unless things have changed, the Welcome only takes cash. Happy Travels!

Since you're having trouble finding availability and time is short, I suggest a website that allows you to search by date.

www.venere.com is a good one, which also provides ratings/reviews by past guests. I just did a test for 9/15-9/19, and it pulled up 114 options. (Which you can then sort by price, rating, arrondisement, etc.)

Bonne chance!

I stayed at Hotel Andrea near the Hotel de Ville several years ago. It's small and plain, but it was very clean and in a good location. Metro stop just around the corner, walking distance to a lot of good stuff.