Car rental rates wary greatly, depending on season, length of rental, and type of car. All the generally known companies are represented and there are local companies too. You can get an idea of the cost if you make a search for "Costa Rica car rental" and put in your specific dates etc. Nearest airport is Liberia (LIR). Most of the year, and definitely in August, you will want to have 4x4 and not the smallest vehicle.
August is in the middle of the rainy season, but it is not the worst month at all. August 2012 was sunny and way too dry. In an average August - not that the weather has been very average the last few years - mornings are very sunny. By lunch time, the clouds start to form and a couple of hours later, it starts to rain for 2 - 3 hours. Often there is a break until mid-evening when it starts to rain again. Most days, it is raining when we go to bed and sunny when we wake up.
The hottest months are April and May with temps in middle to upper 90's. When the rain starts in middle of May, it starts to cool down a good bit, but is still muggy, and it gets cooler until October which we consider cold - all the way down to mid-70's! <img src="http://www.traveltalkonline.com/forums/images/graemlins/Yikes.gif" alt="" />
Our place is not beach front, but we have a great view of the ocean. It takes us 12-15 minutes to walk down to the beach, about 20 to come back up the hill.
No beaches in Costa Rica are officially "nude" (public nudity is not allowed), but, as FreshAir reported in the first post, I believe, there are beaches that are deserted enough to allow for a stroll in the buff. Depending on how much it has rained, crossing the river to the beach mentioned might not be possible.
.
