If you have more time, consider taking a day trip outside of the cities. There are many beautiful destinations just a short train ride away from each of these major European cities. For those with less time, consider cutting out one or two destinations. It's better to fully experience and appreciate a few places, rather than feeling rushed through many. Remember to always check the opening and closing times for each site and book tickets in advance when necessary. Lastly, make sure to try some local cuisine and soak in the vibrant cultures of Barcelona, Milan, and Lisbon.