Ecuador is divided into three distinctive climatic zones (four, if you count the coast and Galápagos as separate).
Quito & Highlands: Quito is broadly representative of the climate in the Highlands region. Weather is cooler and quite rainy in the Highlands around now, meaning that, despite a hiatus over Christmas and New Year, low season is well on its way. Myriad festivities compensate for the raininess.
Oriente/Amazon: Ecuador's Amazon climate varies because this is a vast region. December sees some of the warmest and driest weather in Parque Nacional Yasuni, where many birds come to sun themselves in rainforest canopies.
Desert Coastal Strip & Galapagos: This a good time of year to visit Ecuador's coastline. It's getting warmer around now, and sunnier, despite the sporadic showers (usually in the afternoon). Even more upliftingly, the fog that has hung over the coastline for much of the last few months is gone, replaced by clear skies.
In the Galapagos, this is the shift from cool/dry to warm/wet season, so if you are visiting this month you will get the best (and worst!) elements of both sorts of climate. Overall though this is one of the warmer months in the Galápagos, with calmer seas for boat trips, and lands looking lush from recent rain.
Crowds & Costs
Crowd numbers are increasing in Ecuador throughout December. This is mostly due to the month's many festivals and people vacationing over the Christmas and New Year period. The beginning of the month can likewise be busier because of visiting Americans on Thanksgiving vacations. At these times, prices are high and hotel rooms at a premium. It may also be much harder to book your place on certain tours if you leave it until the last minute.
There is a brief dip in visitors in the second week of December. By region, the Highlands see higher visitor numbers despite the worse weather because of all the month's festivities, the Oriente continues to receive lots of travelers, and the coast and the Galapagos are getting much more popular as high season in these regions approaches. It's particularly nice on the coast this month: the big crowds haven't arrived yet, but the weather is lovely for some beach time and swimming.
Where to Go
Quito & Highlands: Cool, wet weather does not matter if you are exploring Ecuador's superb cultural heritage, and the fabulous colonial churches and museums of Quito or Cuenca are great to explore right now. Quito particularly is a must-visit this month: enlivened by many festivals and with a treasure trove of colonial architecture to explore. The other great destination to check out in the Highlands is the serene travelers' hangout of Baños, with enough outdoor activities to amuse whatever the weather and important festivities in the middle of the month.
Oriente/Amazon: The Oriente climate varies greatly. But overall it's dry by jungle standards, and so the obvious choice of activities (and that which will be offered on many jungle tours) is seeing the jungle from the land: on rainforest pathways and from canopy walkways, in places such as Parque Nacional Yasuni, which has a wide range of lodges to choose from that suit all tastes from luxury to rustic. See here for a list of the top wildlife lodges in the Ecuadorian Amazon.
Desert Coastal Strip & Galapagos: Get some pre-high season sun-bathing and swimming at coastal beach towns like Mompiche, Canoa and Montañita. Or travel to smart, bustling beach towns like Bahia de Caráquez, from where you can go on dolphin-watching tours and explore little-visited beaches.
And, before the hot coastal weather really arrives, go to the place that makes the finest Panama hats on the planet at Montecristi (a Panama hat's proper name is indeed a Montecristi). On the Galapagos, make sure you visit the island of Española, where the rare waved albatross is active this month.
What to Do
Quito & Highlands: Start at Quito's pretty central Old Town squares, Plaza Grande and Plaza San Francisco, for an insight into Ecuador's wealth of colonial architecture. Or visit beautiful churches and world-class museums in Quito or Cuenca. Buy memorable Christmas presents with the ultimate shopping trip at Otavalo, home to one of South America's most important handicrafts and produce markets. Or simply celebrate this month's many festivities.
Oriente/Amazon: This is the perfect time to explore the jungle on its pathways. Some of these lead to canopy walkways that are great for watching the forest from above (in the drier sunnier weather birds love sunning themselves on the canopy). You also see different wildlife to that on the rivers: that which lives on or in the trees, such as snakes and spiders, plus some interesting plants.
Some lodges run ethnobotanical tours at this time of year which explain insect and plant medicinal uses, and how tribes have utilized the rainforest's rich variety of plant life for food, healing and practical items for centuries.
Desert Coastal Strip & Galapagos: Get some beach life before the mad rush to the coast starts in earnest. Boat trips (to see dolphins at Bahia de Caráquez or to Isla Portete from Mompiche) are good around now, with warmer weather and clearer skies.
Snorkeling always has something amazing to offer in the Galapagos, and around now it could be watching sea turtles or seeing giant tortoise eggs hatch. Also look out for the waved albatross, endemic not just to the Galapagos but to one island, Española: the birds' fledglings are developing and learning to fly around now. Plant-lovers will also relish the islands being in bloom from December's rains.
Fiestas de Quito/ Founding of Quito Festival: December 1-6. The founding of Quito is on December 6 but in true Ecuadorian style, the celebrations get going much, much earlier. The whole of the first week of December is full of revelry. Open-topped, colorfully-bedecked buses called chivas careen through the streets full of dancers, bullfights are held, and live music, both traditional Andean and modern rock, fills the streets.
Fiesta de Baños: December 16. The big annual celebration in Baños, marking the date it became capital of its canton (a canton is similar to a province). All you really need to know is that there will be much dancing, drinking and parading on this date!
Fiesta de Inocentes: December 26-31. If you are experiencing a post-Christmas, pre-New Year lull then Atuntaqui in Imbabura province could be the place to go. Day in day out over this period there are costumed parades, burning of effigies and prodigious dancing and partying.
Año Nuevo/ New Year: December 31. Quito and particularly Guayaquil have vibrant festivities to see in the New Year: expect lots of fireworks, plus the usual merry-making.