You'll love Paris, great choice for this trip. For an anniversary holiday I would choose a regional Italian option. Italy because the people and the culture are every bit as engaging and passionate as the French, and the options I'm going to seell you on are all within easy reach of Paris.
1. For romance, Venice! This is an experience you'll remember forever, no matter how many future holidays you take together. Magical, and plenty of walking, exploring, catching vaporettos around the canals and to other islands to fill a week. Go!
2. For stunning scenery, and beautiful walks, Cinque Terre. Five small hillside villages by the sea, all connected by a rail line. From Paris: light to Genoa or Pisa and a connecting train. I stayed in Riomaggiore last year but a y town that takes your fancy would be equally blissful.
3. Are you both 'foodies'? Piedmont, in the north west (i.e. close to Paris!) is the home of the slow food movement, and they have a food festival from mid-October till mid-November. I'm going in late October, can't wait! http://www.fieradeltartufo.org/index.jsp?idProgetto=2
4. Tuscany - wow. Spent about 10 days using Florence and Siena as my base here last year, two amazing towns, small enough to walk around, so much to do, especially if you like your art, Florence. Truly you could pick any of the incredible Tuscan towns - and explore local wineries on a day trip. Direct flight to Florence, bam!
If it were me, in your circumstances, Venice. If you make it have a Bellini in Harry's for me.