Do both. Take the tour that starts at the visitors center. Then walk around after. And yes you can walk around. If there is an area of building you can't go in, it will be labeled. The visitor center is a great place and has tons of stuff to see. The USNA museum is also a great place. Admissions gives briefs on certain days, not sure if they do them Saturdays, especially with a 12 PM football game. Campus will get real empty tomorrow once the brigade marches over. Downtown Annapolis is another great place to walk around and get some crab dip, ice cream, cookies and other great food.