Galapagos is good to visit all year round but there is more sun and warmer water from December to May.
Since the year 2000 the Ecuadorian government adopted the US Dollar as the national currency so if you already live in a country where you use them you can just bring them alone, otherwise make sure you exchange money before arrival.
To visit the Galapagos Islands there are no vaccinations neither required nor recommended. If you are anyway planning to spend more time in Ecuador, especially in the rain forest or Amazon area, vaccinations and immunizations strongly recommended. Since this varies from time to time please consult with your local health office (or the Institute for Tropical Diseases) several weeks before your trip.
A passport is required, most North American and European countries citizens do not require a visa in advance ,this is issued on arrival for a maximum of 3 months. Your travel agency can advise you of any visa requirements for your particular country when travelling to Ecuador.