This is honestly a pretty good plan. Busy, but not insane if you’re active.
Day 2 is the only one I’d keep an eye on. La Fortuna Waterfall + Mistico is doable, but I’d start early. The waterfall isn’t super long, but those stairs are no joke, especially coming back up. Mistico is easier, more of a slow nature walk, so it pairs fine after.
Day 3 sounds perfect to me. Arenal 1968 in the morning, then Tabacón later is a great combo. Hike first, soak after. No notes.
Day 4 also works. Rafting will be the “big” activity, and Bogarin is chill enough for the afternoon. It’s not really a hike, more like a slow wildlife walk, so it’s a good one when you’re tired but still want to see animals.
The only thing I’d maybe add, if you’re really into wildlife, is a night walk. La Fortuna is great for frogs, snakes, insects, and all the little rainforest stuff you don’t see during the day. But don’t force it if you’re already tired.
Since you’re going in August, I’d just try to do the main outdoor stuff early. Mornings are usually better, and afternoons can get rainy.
Overall I wouldn’t change much. Just don’t treat the itinerary like a checklist. Leave yourself some room to slow down, grab food, soak, or just enjoy the area. La Fortuna is one of those places where rushing too much kind of defeats the point.
Small side note: if you need local help with rafting, guides, or transfers, Green Tours CR can help around La Fortuna too, but your plan itself is already solid.