I've been all over Costa Rica but haven't gone to the other countries you listed (yet). Highlights of Costa Rica for me are Cahuita (small town on the caribbean side) and monteverde (kind of touristy but a neat place none the less). My only recommendation is to eat as many casados as you can. They are basically a "traditional" costa rican breakfast/lunch/dinner plate. They are typically much cheaper than other meal options and usually have plantains, rice and beans with an egg or meat depending.
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