Nope...aint gonna happen
You can't day trip from Arenal to Monetverde, its a 4 hour drive each way. You need two night at Monteverde. It will also be very wet, foggy, misty and not much to see in Aug...skip it. Its touristy too.
Its 3 hours drive from SJ to Arenal, Its 4 hours drive from Arenal to Tamarindo, via going around the lake which is very nice.
Driving from Tamarindo to Montezuma will take 4-5 hours, via the ferry in Puntarenas to Tambor. The road east of Nicoya Rt 21 turns into a single track trail for 20KM, not a lot of fun in rainy season either. Why are going all the way to here anyway, its a small town and small beach to swim?
Montezuma to Manual Antonio, figure 5 hours, via ferry Puntarenas.
Manual Antonio, why here? Its a small park and very touristy, its also very developed and and a so so beach...its very out of the way.
If you want adventure, I suggest you head to Arenal 2-4 days and do side trips to Tenerio/Celeste, or Cano Negro. Then return car in La Fortuna and take a shuttle to La Pavona and boats to Tortuguero. Far more nature and wildlife than anything you have listed. Then from there, head via boat to Moin, and taxi hire to Playa Cahuita and the park there. Rustic and spicy afro caribe food, and funky music too. Then tax down to Pt Viejo area and the beaches south, far nicer than most the beaches on Pacific side, outside of Tamarindo/Nosora or Mal Pais.
Then raft the Rio Paquare back to SJ from caribe side and get dropped in SJ. That is adventure.
The 3 most touristy and visited areas in the country, are Arenal, Monteverde, and Manual Antonio, so not sure where the "adventure" is on your plan, other than a lot of driving two lane roads.
The other area for "adventure" would be the Osa, and Corcovado, the best park in country, region and hemisphere for bio diversity.
The caribe side tends be drier in August too, then the Pacific side. The Osa is pretty wet though, as is Montezuma area...
Keep in mind its rainy season, and expect afternoon showers and also heavy rains in mountains. But its a great time to visit, everything is very green and lush and the rains cool off a hot day.
Also, be aware of Dengue, and Zika issues, especially on coastal areas/beaches and lowlands.
