Many cities such as Paris, London, New York, and Rome offer city passes which include admission to popular museums, galleries, and attractions. Some include transportation passes. Most allow you to skip the long check in lines and go directly to the will call or purchased ticket lines. Check to see how many days you will be in that city and then purchase your pass for one day or for multiple days.
Almost all will save you money and time, allowing you to spend more time visiting your favorite sites.