Honeymoon in March / early April?

You mentioned honeymoon, art, food, and late March/early April. Europe is a wonderful place to go for art and food. Italy probably has more art per square kilometer than any other country.

Romance/honeymoon: Paris is romantic any time of the year.Some think Venice is very romantic, others think it's too "touristy" or expensive.

Art: the Orsay and Louvre in Paris (just to name two of many); the Uffizi and The David in Florence.

Food: You can get good food, or bad food, anywhere. France takes food seriously. Italy just has good food, they just don't talk about it the way the French do.

Weather: Not sure how much weather matters to you. That time of year in Northern Europe may be like Portland weather that time of year. Places within a hundred miles of the ocean or English Channel will have weather that is moderated by the large body of water; interior continental locations like Germany or Eastern Europe will tend to be colder in late March. It's difficult to generalize about the weather in such a large area as Europe, and maybe weather doesn't matter that much in your plans. April is a transition month in most areas.

You have a lot of trip planning to do, and you can make that part of the fun, especially if you do it together.

You didn't mention whether this is a first trip to Europe; in what I said above, I was assuming it is.

That's a great time of year to visit a city that is otherwise overwhelmed by tourists. My first trip to Paris was in March and I'm so glad I saw the big sights then, because on subsequent trips in May, September, even October the lines for sights like Notre Dame and Ste Chappelle were incredible. Paris would be high on my list but Rome (check to make sure you wouldn't be there over Easter, though), Florence, Barcelona, would be great too. I'd avoid countryside destinations since the weather can be iffy--in cities, there's always indoor stuff to do.

I agree that spring (albeit you are going early) is a wonderful time to visit big cities in Europe, especially further south, although the northern larger cities are great as well. We were in Paris in April of this year and had a great time with limited crowds and romantic weather (which in my opinion is in the 70's). How about doing London-Paris-Amsterdam, or Rome-Siena-Florence-Venice (but in Italy, beware of Easter/religious holidays for that time of year that may impact your trip).

I think that anywhere is Spain would be a fantastic choice, particularly Barcelona and surrounding areas. Amazing food, unique art and wonderful culture.