I think my wife and I have this down pretty well, so here’s our guidelines:

1. Figure out some structure. We usually plan one “thing” per day. Whether that’s catching a train between cities, a particular museum, or a guided tour. This helps with pacing when you are there because you don’t have to think too much day to day, but you won’t feel like you wasted a whole day.
2. Figure out food options. I usually make a Google Maps saved list of dozens of different kinds of restaurants in every city. The goal here isn’t a plan, but simply to have good options no matter where in the city you end up. You will have less than one dinner per day of travel after you consider traveling days, so don’t waste it on some tourist trap that you happen to be nearby when the time comes. I’ll usually make a dinner reservation for every other night to make sure we get some incredible meals.
3. Naps. It’s vacation, just plan on taking a nap everyday. Our first trip was together was to southern Spain and we’ve just decided that siestas are for us. This also helps with jet lag, staying up late to do local stuff, and having something that you won’t feel bad about canceling if something comes up.
4. Self-Guided tours on the first day. If you are Americans traveling to Europe, I’d recommend the Rick Steve’s app and then splitting a pair of AirPods together as you walk around. He does the whole look here, walk here, turn left tour thing, but it’s self paced. We try to do this the first day we’re in a city so we get an idea what the major areas are. Self paced is nice because he’ll say something like “this is a great coffee shop” and we can just pause it and grab coffee if we want. Split the AirPods so you can really hear your surroundings and the tour is something you share.
5. Any plans you make are just so you know your options. If you plan on taking a train between cities, look at when the next train is in case you have to miss it. Same with dinner reservations or museums. If it doesn’t feel fun or convenient, you’ll want to know what your alternatives are so it’s never “something or we read in the hotel all day”. Think about “it’s raining, so we’ll go to a museum instead”. Rick Steve also does museum tours.