You should do the day trip to Arenal volcano. Arenal lake has some good shots.
That whole coast area has nice beaches with great sunsets. Coco, Flamingo, Tamarindo, remember it is a small country so visiting other areas is easy. You can do surfing shots in the area, Flamingo sticks out in my memory, constant big curlers there.
Tamarindo has lots of surfers. The beach is a bit rocky in parts. The south has dangerous waves breaking over the corals. The north has a worthwhile river trip with crocs, lots of birds in the mangroves, etc. The one eyed boatman gives a great trip. A large group of pelicans sweeps over the beaches every morning. There are turtle beaches a bit farther south. Iguana foreground with beach background works!
It is a bit dry even though it is on the coast. Guanacaste is considered the desert part of the country. If you want big rainforest you have to go to the interior. The trees around there are a bit scrubby so its beach shots, town shots, surfer, lots of yachts, big mansions etc... The palms off the beach are nice of course.
The bonus is fewer mosquitoes in the dry country.