Eurostar would definitely be my preference. Booking well in advance should see you offered a return fare of £69 per person but, since it's a special occasion, you might want to upgrade to Premier class. That would get you a reasonable breakfast on the way out and a decent dinner on the way back (with excellent Champagne and/or wine).
The Latin Quarter is packed with loads of really good (but surprisingly cheap) restaurants. Simply take a Metro or RER train to St Michel-Notre Dame station and wander around until you find something you like the look of.
That station is right next door to Notre Dame Cathedral, which is well worth a visit. Further, it's close to the eastern end of the Batobus route. The Batobus is a hop on/hop off boat service, which runs to the Eiffel Tower (with stops for other attractions, such as the Louvre and the Musée d'Orsay along the way). It offers an excellent way to see the sights.
When not travelling by boat, the RER train services provide the best way to get around. They're like a limited-stop version of the Metro, offering faster (and frequently less crowded) services. You can buy 'Paris Visite' tickets, offering one or two days unlimited travel within Zones 1 to 3 at the Eurostar terminal at St Pancras. (They also offer discounts to some attractions). They're valid on bus, tram, Metro & RER services. (Alternatively, you can buy 2-Zone Passe Illimité passes, which is what the locals use, when in Paris).
Batobus info:
http://www.batobus.com/english/index.htm
Public transport guide:
http://www.ratp.fr/en/ratp/r_50240/paris-public-transport/
Official tourism site:
http://en.parisinfo.com/
Chris