Yes you can just show up and walk around the yard with a valid photo ID. However there are tours and info sessions i believe every day, you can check that on the website, but for a CVW you will need to request that from your BGO.
Candidate Guidance, not BGOs, determine who will be offered a CVW. BGOs can recommend that a student be offered one, but I'm honestly not sure how much weight that carries.
Realize that not every well-qualified candidate can be offered a CVW as this would entail literally thousands of visits each year which is logistically impossible for USNA. I don't know what specific criteria USNA uses to determine which candidates are offered visits -- I do know that it is GENERALLY a good thing to be offered one and NOT necessarily a bad thing not to be offered one.