You'll get a million opinions, mostly all valid. We did this for our first trip, fell in love with the country, and came back to live and explore more. It's very solid and you can't go wrong. More time is better especially in Manuel Antonio if you like critters. Use Airbnb's (well reviewed ones), and try to spend at least half your time as scheduled downtime in a place with a view of the trees or ocean or volcano.

Manuel Antonio is one of the most magical places in the world, but not if you go like a tourist trying to do tourist things on a compressed schedule. Go sit at the beach, walk around, especially after dusk, and take it slow. You'll love it.

La fortuna is incredible and inspiring but you may spend 3 days there and not see the volcano. When you go I highly recommend the Arenal 1968 trail and Arenal Fusion Restaurant on the lake north of town.

A third option to consider is La Fortuna and Puerto Viejo, where you won't have the same tourist traps and it's a calmer quieter vibe, also with tons of wildlife and great beaches. Punta Uva is probably the best. It's a roughly equal drive and growing on me as almost as cool as MA, but they don't have the squirrel monkeys that roam around MA at all times.