We stayed last year at the Hôtel Napoléon (40 Avenue de Friedland) which is 1 block from the Arc de Triomphe. Other great hotels location wise is the Westin Paris and the Grand Intercontinental.
Romance wise, I would definitely recommend a Seine evening dinner cruise (Bateaux Parisiens River Cruises) - an unforgettable experience. Get the upgraded package which allows nicer wine & entrees, not to mention a table at the front.
And, last but not least - go to Willi's Wine Bar m(13 Rue des Petits Champs) for some great wine and food.