I would suggest limiting it to 2 locations as travel around Costa is slow (windy, hilly, single-lane roads, and sometimes dirt roads).

Also, I think you can do the Arenal area in <3 days. I did 4...

Went there with the wife a couple of years ago. There's plenty to enjoy about a place that is sand and sea. But the beaches have way more to offer. Some shores are lined with black or pink sand...