Unfortunately, it's hard to think of many 1TS credit restaurants that would be nice for an anniversary dinner anymore -- most of them are so unbelievably crowded and noisy, now, with the Dining Plan, that there's not a hint of ambience in most of them.
Perhaps Kona Cafe at the Polynesian? Maybe Olivia's at Old Key West? We haven't been there in a long time, but it was nice last time we were there.
If they like buffets, Boma might be nice -- AKL is a beautiful hotel and it's fun to walk out and see the animals in the savannah -- especially interesting at night if you find where they're letting people use the night vision goggles.
A lot will suggest Le Cellier in Epcot, although we weren't that impressed when we went and it's apparently hard to get reservations.
Are they the type that would like a character dinner? I think character meals are something of an acquired taste and unless there are children in the party it's a taste I really haven't acquired! But, others enjoy them. I'd stay away from Chef Mickey's in any event -- too frantic.
In World Showcase, I'm not sure I'd tackle any of the usual 1TS suspects. I'd consider Marrakesh in the Morocco Pavilion. It's usually not as crowded as the other places, was pretty good last time we were there, and there is usually some type of entertainment featuring Moroccan musicians and a belly dancer.
Just beware, any 1TS place you go to is likely to be extremely crowded and crawling with children. There, I've said it! You just have to deal with it, or try a 2TS place (and no guarantee it won't also have lots of kids, too) or eat off site (probably not a good idea if you're on
DDP).