Hello and welcome...
First things first - you'll find a wealth of personal opinions and experiences in the Central America portion of this board, found here: http://www.travelpunk.com/boards/index.php?showforum=38
A handful of us have had stints in C.A. and have posted our experiences there.
Next: Best places to go and things to do really depend on what you want out of the trip. CA's a huge destination for eco & adventure tourism, so there's all that, but it's also a great place to go and immerse yourself in another culture and take the opportunity to do an intense Spanish program.. Or perhaps you just want to surf and be a beach bum the whole time. Or just party on the cheap. There are options galore.
Figuring out what sort of things you want to do will determine your costs, how long it will take you to do those things in each country, and the route you might want to take.
As far as airfare goes, it's tied in with the season. Being a tropical part of the world, our summer is their rainy season, so just before or after that is pretty ideal in terms of price and weather. Of course, if you're bigtime into stuff like rafting, then the rainy season is a better time to go, since the waters will be higher, and if you're a beach bum, then peak season is for you.
Your first step is to figure out what you want to do. And checking out the linked forums might give you an idea of what's out there.