There are so many locations that it is hard to pick one. IMHO, it also depends on what the couple wants. If they have already been in Paris, you may want to find less known places. If this is a first for them, you've got plenty of very recognizable landmarks that offer lots of opportunities. Jardin des Tuileries, Jardin du Luxembourg offer "green" locations at the very center. They are smaller than, say, Hyde Park, but you'll have dozens of typical "French" locations. The "Pont Neuf" can offer nice, if a bit standard, opportunities. I'd take the tube, exit at st michel, pont-neuf, chatelet and walk around a bit: you could spend a week in a 1km radius if you want to see everything. Montmartre is also a decent place for "typical Parisian" photography. "Champs-Elyses" aren't worth the trouble imho.
Unfortunate reality: if you look like tourists, you _will_ be ripped off on every occasion. Not typically French, London's taxis also have a tendency to take the longest path. Typical attitudes to watch for: random markups on displayed prices in pubs and restaurants, change not coming back if you pay cash, taxis who don't start their counters and don't accept credit cards but only tell you when you have arrived, etc...
I've have been in the "change not coming back" situation several times. Typically I am with an English speaking friend and the waiter hears us talking in English. We pay cash and the waiter never returns. At that point, I switch to French, find the guy and asks if he isn't forgetting something... I've had the "ooops, sorry, I thought you were American" response (and my change) more than once ;-)
I'd like to add that outside that particular central area and the areas around the main stations, that "ripoff" attitude disappears completely.
While there are nice places on the edge of Paris, I wouldn't bother given the many choices in the center.