DD validated several classes (English, calc, chem, French) as a result of either AP score or validation test results the first few days of Plebe summer. There is a matrix on the USNA web site that shows the validation requirements for various classes. It's under academics. It's also in at least one old thread, probably several, on this site.
Validation and workload: Validation doesn't make a huge difference to workload, since everyone must carry at least 15 hours/semester, even if they have completed all requirements for their degree. Still, 15 hours can be a lot easier to manage than 18 or 20 hours. What validation does do is open holes in your schedule to take interesting electives, or to do research during 1/C year, or to start a Master's degree.
A word of caution to those who think it will be easier to NOT validate: Do not think that you can get by without proper study, even if you're taking "a class you've taken before."