We went last July for 10 days. Our boys were 15 and 18. I planned it myself.
We flew into San jose, got transporation to Manuel Antonio and stayed at Tulemar (highly recommend, but it was a little pricey, check it out on Tripadvisor) for 5 nights. Lots of monkeys and sloths, and some other stuff (koati, iguanas). There's a nice beach there also and it's not far from the park. We did the ADR all in one adventure (a little too adventurous for me but guys all liked it). Guys also went out inshore fishing with a private boat and had a blast. At Tulemar, you also get a private concierge who drove us to the store and ATM, and could help with any questions/planning before and during our trip.
We went to Tortuguero for 3 nights(two would have been enough) afterwards which is extremely rustic, but amazing wildlife (saw sea turtles come out of the ocean and lay eggs), tons of giant iguanas, monkeys, rediculously large bugs

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There does seem to be some great areas that we didn't get to see on that trip, but everything was so far apart that we just focused on two (and San jose as a stopover point)
I wasn't comfortable driving there so I found a driver on tripadvisor who was great. He drove us to each location when we relocated. Though I've read many people have no issue and are very comfortable renting a car.
This was a pretty expensive trip for us, we usually try to keep trips around $4k all in, but this was DS18 graduation trip and he chose Costa Rica. Subtracting out the airfare(that was about $2k but will be different depending on where you are coming from), I think we spent about $6k for all lodging, driver, meals(including hiring a private chef one night), excursions.