hey adrienne. Of particular interest on a sunday might be "brunch on the beach" The city hosts this activity, I believe it is every Sunday morning through afternoon at Waikiki beach. Its more of a small festival.
Also on sundays, the Royal Hawaiian hosts a pink champagne brunch. This could be nice.
These are the only recurring "events" particular to Sunday; but there are plenty of other possibilities.
Of restaurants, I have heard La Mer is about as good as it gets in Honolulu. Its French and has a spectacular setting. Allen Wong's also has a good reputation, and it is Pacific Rim style cuisine. I'm sure you can find more information on restaurants via the web easily.
There are plenty of romantic opportunities in Hawaii. Sunrise is best seen from the Leeward side (Kailua Beach.) The sunset is rarely a disappointment in Waikiki. Hawaii is best appreciated by getting into or on the water. But, if you don't want to get wet- try a boat ride/day cruise. Or better yet, a dinner cruise. Hey, that might be an idea. I haven't done it myself, but its worth checking into..
If you want to know what I'm planning personally, check this out...
https://www.pricescope.com/community/threads/proposal-idea-whatdya-think.6806/
anyway, hope this helps..
Jp