All of the countries in Central America require a passport valid for at least six months from entry into the country. If you're traveling into a Central American country from an area with any risk of yellow fever (like Panama's Kuna Yala region) you'll also need to provide a vaccination certificate. Visas are not needed in most countries unless you plan on extending your stay for more than 90 days.
Some countries are part of the Central America-4 (CA-4) Border Control Agreement and have more flexible travel regulations. Under this agreement, eligible foreign visitors can travel within El Salvador, Guatemala, Honduras, and Nicaragua for up to 90 days without completing entry and exit formalities at border checkpoints. The CA-4 entry agreements can only be extended once, and additionally, travelers can leave the member countries for 72 hours and return to apply for a new 90-day allowance. If they overstay without getting the appropriate extension, they’ll be fined.
All travelers need a valid passport to enter Costa Rica, ideally with more than six months left on it and plenty of blank pages. Visas are not required for USA, Canada, Australia, Britain, and European Union nationals if they're staying fewer than 90 days. If you intend to stay longer, you must buy a tourist visa for $160 USD and exit Costa Rica for at least 72 hours before re-entering the country. Technically, travelers must be able to prove they have more than $500 USD in their bank account upon entry, but this is rarely checked.
To enter Honduras, all travelers need a passport that's valid for at least three months after the date of entry as well as a return ticket. As part of the CA-4, Honduras allows tourists to travel to and from Nicaragua, El Salvador, Honduras, and Guatemala for up to 90 days without dealing with immigration formalities at the borders.
All travelers need a passport to enter El Salvador, valid for at least six months past the date of entry, as well as a return ticket. Nationals of Canada, Greece, Portugal, and the USA must buy a tourist card for $10 USD on entry, valid for 30 days. Australian and British nationals do not need a visa.
All travelers need a passport to enter Panama, valid for a minimum of six months. Occasionally travelers might need to show proof of a return ticket and at least $500 USD in their bank accounts. Nationals of the USA, Australia, and Canada are issued tourist cards for stays of up to 30 days. Because the tourist cards only cost $5 USD, they’re often included in international airfare. Check with the airline ticketing agent when you get to the airport to see if your plane ticket already has the tourist card covered.
All travelers need a passport to enter Guatemala, valid for a minimum of six months. You don't need a visa if you are staying less than 90 days, under the CA-4.
All travelers need a valid passport to enter Belize, good for six months past the date of arrival. While travelers are supposed to have sufficient funds for entry—sufficient meaning a minimum of $60 USD per day of your stay—they are hardly ever asked for proof. All tourists and non-Belizean citizens are required to pay an exit fee of $55.50 USD; this is typically included in airfare for American travelers. If it’s not included in your airfare, you’ll need to pay the fee in cash at the airport. For those leaving Belize at the Guatemala or Mexico border, the exit fee only costs $20 USD.
All travelers need a valid passport to enter Nicaragua; for all countries except the USA, the passport must be valid for at least six months. Travelers can obtain tourist cards upon arrival for $10 USD, good for up to 90 days. You will also have to pay a departure tax of $32 USD when heading home.