Hello. We will be traveling to Arenal in Dec 5-12. How have you all experienced the weather in December ? Rainy still?
It is the tail end of the rainy season and Arenal in general gets a fair bit of rain year round. The weather there is unpredictable. It can change a few times a day. Pack waterproof items (a light rain jacket and/or a thin portable rain poncho that easily fits in a pocket or a day bag) and footwear with good non-slippery sole, which you don't mind getting wet/muddy, and which can dry relatively fast for the next use. Basically, be prepared, but hope for the best - clear views of the volcano and sun. Most activities continue in the rain. Ziplining will likely be cancelled if there is lightning.
I have been near Arenal several years mid December. The amount of rain is unpredictable, more so than other months. Last year we had some rain every day, but not for all day. Previous years we did see some sun but had rain on other days.
If you're there a full week that increases your chances of having some nice weather and clear skies for volcano views. If you're lucky that way, consider day trips to Rio Celeste and Cano Negro for a boat tour.
A full week is a long time just for the area around La Fortuna.