The Caribbean Coast will still be in the rainy season in August, though it is towards the end of the season. The drier season doesn’t begin until September and goes through October. In general, the weather on the Caribbean Coast is much harder to predict so we wouldn’t necessarily plan your whole trip around that. There are lots of other places in the country on the Pacific side that are still nice in mid-August. You’ll get some rain for sure, especially in the afternoons and evenings, but it shouldn’t be a washout altogether.
If you’re looking for itineraries, you could check out our book Top 10 Costa Rica Itineraries. It doesn’t get into specifically places that are best for visiting during the rainy season but will give you some ideas for how to piece together an itinerary. If you want more customized help, we could also design you an itinerary for your trip that will help you decide where to go (keeping weather in mind), what to do, how to get around, etc. There’s more information on our Itinerary Help page or just let us know on this thread if you’re interested.
Best of luck planning and don’t worry too much about the weather. It’ll be a little rainy here but you’ll have plenty of sunny days!