www.CostaRicaTicas.com

Good time of year to go to Guanacaste. Check out Playa Samara.

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"With foxes, we must play the fox." ~ Thomas Fuller

Prior posters are correct, those are nice places, but Quepos will have the edge on the fishing; plus beautiful beach, one of the nicest in CR, lots of good restaraunts/hotels of which you'll have your choice--CR in general is pretty dead that week, rafting and other tours nearby, but the volcano thing u should probably do as a day trip out of SJ. Nice time to do the non gulch thing--very few tourists and tico's at the beach towns that week. Start computer searching about Quepos/Manuel Antonio and you'll have an abundance of info about the area.

I went on a whitewater trip on the Rio Pacuare one Christmas. There are several companies, and several rivers, but the company I went with was "Exploradores". The water level was very good that time of year for me.
A bad thing about going at Christmas time is that everything is premium prices. Hotels are mostly double what they would normally be, and you have worry a little about being able to find a suitable room.
As far as beaches, certainly Manuel Antonio should be on your list. I haven't been to many beaches in Guanacaste, but I have been to Playa Brasilito, and nearby Playa Flamingo and Playa Conchal. I recommend this area (but not Playa Coco), particularly staying in Brasilito with reservations at the Hotel Brasilito would be nice.