Hi! I’m sure you have this sorted by now, but PM me if you want; I lived in Costa Rica for a year and could give you a run down of my favorite resorts and cool places to go off the beaten path.
For starters try: Playa Conchal in Guancaste (sand is made up of crushed seashells and is sooooo white and super blue water– http://www.westinplayaconchal.com/ ), there’s an AI there and it’s driving distance to Arenal volcano, so you could easily do one or two day trips and see the volcano, do the hot springs, and/or go ziplining. It’s close to Playa Flamingo, which is another nice beach.
If you decide to stay in Arenal you HAVE to stay at either one of two places: Observatory Lodge, which is so close to the volcano and the gardens and grounds are AMAZING.
Or try Tabacon Hot Springs resort; all the other hot springs in the area are manmade, this one is all natural, and yes, you can take a dip in them. They also have a to die for spa menu.
Manuel Antonio is touristy, but there’s a reason for it. The park is AMAZING. If you want to get away from that touristy feeling then stay closer to the park in the hills and NOT in Quepos.
Also Tortuguero is lovely. If the Pacific is the ocean, then the Carribean is like bathwater. …Seriously, the water is sooooo warm and just heavenly on the Carribean side. The park there is great and really gives you that “in the rainforest” feeling. Plus the locals get around by canals. CANALS.
But my favorite place to go is a tiny little surf town near the Panamanian border called Puerto Viejo. It has lots of cool tiny little shops, great food, and amazing outlying beaches (Punta Uva and Manzanita) that you can bike to if you want to rent a bike. The Cahuita Park is nice too, and you can do some ziplining there in the forest. Like, actually in the trees, not over a giant valley. There are a few honeymoon-level hotels there, and go to one of those; skip the surfer bungalows.
Playa Hermosa is a sleepy little black sand beach town. Super nice area, very quiet. Also there are macaws like CRAZY in that area. They mate for life so they are constantly flying across the sky. We went for Valentines day and it was amazing and so romantic. Pack LOTS OF SUNBLOCK because it is sunny there.
I’m sure you’ll have an amazing time. If you want to stay away from touristy areas avoid: Jaco, Tamarindo, and don’t spend more than a night in San Jose on either end of the trip. Villa Sol’s another pretty nice AI in Guancaste and is off the beaten path, but I feel like a lot of retirees have villas there… mixed vibe but nice overall.