Hi Maya,
That time of year is actually the driest for the Caribbean side of Costa Rica so Puerto Viejo should be just fine and you might want to even consider adding more time there. Tortuguero as another destination maybe? Arenal might be a little bit of a risk, you could end up not seeing the volcano at all so just keep that in mind. For more info about traveling during the rainy season definitely check out our Weather in Costa Rica post and Why You Should Visit Costa Rica in the Rainy Season. There is lots of good info in the comments of the rainy season post too! Our biggest advice is to be flexible with your activities so that you can change with the changing weather.