Wanted to spend some time in the rainforest as well as relax on a beautiful beach
That's rainy season, so it's going to be hit or miss for things. And when I say rain, I mean flooding so make sure wherever you are ending your trip, that's the part closest to your airport. (I'm not sure if it's rainy season country-wide or just certain portions.)
I believe all National Parks are close Monday and as of a year or so ago, many (if not most/all) are closed on Tuesday. Don't trust the tour books on the Tuesday part, call the park you plan to go to, the books might not be updated yet.
I love the west coast of Costa Rica (Nicoya peninsula), Montezuma (beach and beautiful waterfalls) and Monte verde (rain forest)would be good places to go.
http://garysaid.com/tags/montezuma/ about 7 years ago