Panama. US dollar is used, huge plus.
You can hike Volcan Baru and see both oceans from the top - way cool. Serious hike though.
Great beaches, and you mentioned beach islands - Bocas del Toro... is heaven. Its where all the rooms are "floating" out in the ocean.
Costa Rica is over played. A week in Panama is too short... but you'll have a blast if you chose it. Plus, extremely safe country too.