For Manuel Antonio, you have to spend some time hiking/seeing wildlife. The national park will be busy that time of year but it’s nice if you go early in the morning on a weekday, or there are other options for places to go. Rainmaker is one of our favorites. Hanging out at the beach is also a must-do- the main beach is great and there’s also Playa Biesanz, which is less popular but gorgeous. Here’s a blog post with wrote with some more ideas.
For Monteverde, you have to hike the cloud forest. The Monteverde Reserve is nice but usually the busiest (here’s a post about it). There’s also Santa Elena, which we really enjoyed, and Curi Cancha. Be sure to hire a guide to see the most- it can be hard to spot animals in the cloud cover. Zip-lining through the cloud forest is really cool and we also loved the hanging bridges at Selvatura. We haven’t done one in Monteverde yet, but it’s also a great place to do a night hike.
Hope that helps. Have a great trip!