The Galápagos are an expensive and time-consuming destination, due to remoteness and need to use boats for travel within the archipelago. A tour of the main attractions takes over a week of a ship-based cruise, and getting to and from the Galápagos takes two travel days from most of the world (often with one day in mainland Ecuador, especially on the way in).
Given the travel time, a week-long cruise (8 days/7 nights in the Galápagos) is a good sweet spot, which covers most, but not all, of the main attractions (cruises can visit 2 out of 3 of north/south/west islands in a week). Longer visits cover all main attractions, and 2 weeks is plenty, but is quite long and expensive. Shorter visits (5 days/4 nights) or land-based visits are cheaper but quite limited, and risk being disappointing; if time or budget allow, consider longer.
From Santa Cruz it is possible to book day trips to the uninhabited islands of North Seymour, South Plazas, Santa Fe and Bartolome. Advance reservations are normally required; however, on occasion you can find space due to a last-minute cancellation the night before.
There are two seasons in the islands: the hot/rainy season from December to June, and the cooler season from June to November.
Each month brings unique climate variations and wildlife viewing opportunities. Peak season for naturalist tours is typically December through May when the seas are the calmest and the weather the warmest. However summer months June, July and August are also very popular as the animals are more active.
September through November is typically low season when most boats will leave the islands for dry dock. For divers peak season is from July to November, when whale sharks can be found at Wolf & Darwin.