Puerto Rico is not a country. It is a US territory. Travel between the mainland US and Puerto Rico is not subject to border controls. It is a domestic flight like any other. So, yes, you can visit Puerto Rico just as you can visit California and New York.
You can visit Mexico and Canada under the "automatic revalidation" scheme. You may not stay for longer than 30 days; if you do, you will need to apply for a new visa.
You will also be ineligible for automatic revalidation if "any of the following situations exists":
- You have applied for a new visa which has not yet been issued;
- You have applied for a new visa and were denied;
- You have traveled to a country other than Canada or Mexico
- You are a national of a State Sponsor of Terrorism designated country, including Iran, Syria, and Sudan. This list can change, so check before you leave.
There are additional revalidation provisions for student visitors, but I have omitted these because they do not apply to you.