tough one, there's so many good restaurants in Curacao. Landhuis Daniel is good and not too far away, probably the closest nice restaurant. (Though a cab even to there isn't cheap because it will probably be coming from town anyway - will you have a car?) It's not real fancy but it's one of the old plantation houses with local character and we've always had very good food there.
Bistro Le Clochard would be a great anniversary place, reserve a table on the water and plan to kill the evening watching the ships come through the channel.
Rijsttafel Indonesia is quite good if you want something a little different, though I think rijsttafel works a bit better with a group. It's in a residential area and a bit tricky to find if you're driving yourself.
Astrolab Observatory is supposed to be very nice, but we haven't gotten there.
The main restaurant at Floris Suites was very good, I forget the name.
Emerald Grill Steakhouse or Portofino at the Marriott could be options, though not especially unique.
You may see a lot of recommendations some places for Fort Nassau, but I don't think it's that good and consider it a tourist trap - nice place to go for a drink and take in the view through.
(You could probably spend more than $100 at some of these, especially if you have a couple drinks.)