This website (I actually use the app on my phone) suggests 1-3 day itineraries for cities all over the world, and allows you to customize. Particularly useful is the additional recommended visit time per site, as well as the travel time by foot between sites. You can tell it what you would like to visit, and the app will suggest the best order in which to visit attractions, based on opening times and distance between.
https://www.visitacity.com/