Visas: Please consult your nearest Colombian consulate to see if you need a visa or any other paperwork prior to your arrival in Colombia. In general, US, Canadian & European tourists do not need visas if staying less than 3 months (although there is a COP$171,000 entry fee for Canadians payable by credit card or in USD or COP cash on arrival), but please double check your particular situation.
Health: check the US Center for Disease Control page to see what vaccinations you may need and to read up on other health issues for the places you will be traveling.
Just in case: No matter where you go internationally, it’s always a good idea to register with the embassy of your home country in case of emergency. US travelers can do so here via the State Department’s STEP (Smart Traveler Enrolment Program).
Money: The Colombian peso is the official currency in Colombia. You can change money (cash) at the airport or one of the official exchange bureaus in the Old City. There is a BanColombia ATM located at the domestic end of the CTG airport terminal and there are many ATMs in the city for withdrawing funds. (As always, no matter where you are in the world, please be careful with any cash you have and be aware of who & what is around you.)
Per this 29 July 2017 article in Cartagena’s El Universal, the following banks do not charge fees for ATM withdrawals (although your own bank might): Banco Caja Social, Davivienda, Procredit, Bancamía, Finandina, GNB Sudameris, Mundo Mujer, Multibank, Pichincha, Bancolombia, Citibank, Colpatria, & Falabella.
There are many online sources for monitoring the exchange rate, such as XE.com.