Not August but this past Christmas for just over a fortnight.
I found Manuel Antonio really commercial when we visited this past Christmas although we stayed in a pocket of oasis in a holiday let (AirBnb) which had its own private pool where we were visited by spider monkeys, iguanas and sloths in the surrounding rainforest.
Besides hiking within the Manuel Antonio National Park itself, I would recommend a DIY visit to Los Campesinos to escape the crowds. You can hike to swim at some waterfalls as well as crossing a single file hanging/swinging bridge! The travel there & back is an adventure in itself - do it in a 4x4! Kayaking amongst the Damas mangroves was another highlight.
The region around Arenal and our stay here and here were definite highlights. Los Lagos had fabulous geo thermally fed pools and nice grounds to walk around where we saw plenty of wildlife. We took a horse ride from the hotel stables to an observatory point of the Arenal Volcano here.
At the rainforest ecolodge hotel, we did the most amazing night walk, a farm tour as well as hiking the property's trails ourselves. We loved our stay here, the DC loved the superb wi-fi and the family bungalows had plenty of space.
In La Fortuna, I definitely recommend visiting the Bogarin Trail where we saw numerous sloths, frogs and birds - more so than anywhere else during our holiday.
We decided not to bother with the La Fortuna waterfall after the fabulous ones at Los Campesinos.
Ziplining - Do the combo package at Sky Adventures which includes a 3 hr walk through the rainforest over wide/safe suspension bridges (compared to the one at Los Campesinos!), a tram ride up the mountain to the start of the ziplines. My DC were 11 & 15 but there were 7 & 8 yr olds in our group. All very safe but fast and thrilling!
We didn't visit Monteverde because we felt that we had scheduled enough driving for a fortnight's holiday and didn't want to feel too rushed or whistle stop toury and besides, we wanted to relax a little which is why we rented an airbnb for Christmas week.
HTH