Oh, I love Costa Rica! On our first trip there I planned it myself, and we spent 4 nights in Arenal and 4 nights in Manuel Antonio, and had an incredible time. Started in San Jose and did a coffee plantation tour combined with a visit to Poas Volcano, and spent some time at the La Paz Waterfall Gardens. In the Arenal area, we did the SkyTrek zipline that
@iujen94 mentioned and that is the longest, high flying zipline I've ever done, it's pretty much ruined me for any other ziplines. We did the "volunteering" activity at Proyecto Asis which is an animal sanctuary for wildlife. DD got to hold and bottle feed baby capuchin monkeys which was a highlight. Also went ATVing with Original Arenal ATV and that was a blast. In Manuel Antonio we spent time at the beach, and did a day trip with ADR Adventure for their 10-in-1 adventure (ziplining, rappeling, via ferrata, 4 wheeling) which is probably the most fun I've ever had in my life.
Our last two times in Costa Rica we stayed at Hotel Punta Islita (Autograph Collection with Bonvoy) where almost all the activities are included with your stay. There is ziplining, horseback rides, yoga, nature walks, and down at the beach you can rent boogie boards or take surfing lessons (these last two are through private vendors so they do cost money). We've gone river kayaking and turtle viewing for an extra fee too. My DH says that is his favorite place in the world. Me, I actually preferred our first trip to Costa Rica because it was more varied and we saw more of the country. But Punta Islita is easy and relaxing.