Not planning to Honeymoon in Disney World, BUT we're doing a short trip this year before our wedding the following year, so it's like a "pre-wedding mini Honeymoon," LOL, regardless I have a bit of experience travelling to WDW w/ my significant other so I'll chime in.
1: Hotels: it's your honeymoon so splurge and go deluxe. As far as "most romantic" go, I would say The Grand Floridian takes the cake. So romantic! But it's not everyone's *cup of tea* so I'd also recommend the Polynesian, which will give off that "honeymoon vacay" feel, and also the Wilderness Lodge, for a bit of a quieter feel. I think even the Boardwalk Inn could be quite nice if you'd prefer to be closer to Epcot and Hollywood Studios. Can't get much closer cause you can walk to them from Boardwalk!
2. Activities: Fantasyland in Magic Kingdom has a lantern photo-op (ala Tangled) that looks sweet. As already mentioned Port Orleans has a carriage ride. Have a treat delivered to your room. Kiss on rides like Haunted Mansion in the dark! And I second the dessert party idea. Maybe not the Fantasmic one, but splurge for the Happily Ever After one, it’s nice. Shop for wedding-related or Mickey & Minnie souvenirs
But also I would just say dining at a nice restaurant and enjoying the hot tub at the resort can be romantic, or walks around the resort or Boardwalk, and grab a cocktail at Abracada-bar. Surrey rides!
3. Splurge for signature dining at least once or twice! I LOVE Be Our Guest for this. Last trip I made a 7:00 reservation, had a relaxing dinner with champagne, and watched Happily Ever After afterwards. We didn't do rides this night so I dressed up
Other favorites are Le Cellier and Monsieur Paul's in Epcot's World Showcase, though I haven't been to either. There are of course wonderful restaurants at the Boardwalk (Flying Fish) and Disney Springs (The Boathouse, Morimoto Asia, etc.) but there's something extra magical about romantic dining IN the parks.
HAVE FUN and take advantage of PhotoPass Memory Maker!!!!