I generally plan the way Chani suggests. I start by looking at a specific place. I research the sights and decide which are my "must sees". Since it's you and husband, you should agree on must sees. I also research a lot of "nice to see if I have time" sights. I then plan 2 must sees per day - unless the sight is unusually large (Versailles needs a full day) or small (Orangerie needs no more than an hour).
A typical day is getting out of your lodging, eating breakfast, making your way to your first must see. Roughly three hours there. Lunch between the morning and afternoon sights. Several hours at your afternoon sight. After that, you have some time before dinner. How do you feel? Do you have energy to visit a "nice to see" sight? If yes, pick something. If not, relax for awhile before dinner. Ask yourself the same question after dinner. Do you want to do something that is open in the evening or are you done for the day? I find trying to force yourself to do more than two major sights a day starts to feel like a forced march rather than an enjoyable vacation. If you're up for another sight, have an idea what's available but don't make it a do or die thing.
Having said all that. Define your must sees, divide by two, and that's how many days you should plan in a city. Like Chani said, if you need two full days, plan three nights with travel days on either side. Three full days? Four nights...
If you looked at my itineraries, you'd think my focus was on nights (where I'm staying each night) but it really starts with giving myself time to explore each destination.
Don't forget to plan travel time. I think a half day from destination to destination is optimistic but doable. I generally plan a full day for travel, with no more than four total hours of transportation (car, train, plane), then see a "nice to see" sight if I have time after I've arrived.
I'd much prefer to limit my stops and see what I want to see at each rather than hurrying through a destination just to get to the next. Each time you move, you're losing valuable time to relocation so you will end up seeing fewer sights, not more.
I also try to avoid backtracking. I probably worry about it too much. Any backtracking, to me, is completely wasted time. So I put my destinations in a logical order, fly into the first city and fly home from the last. If I fly roundtrip from one city, I plan the trip as a loop rather than an out and back(track).