Hi Kate....tha's a toss-up...but, 1st time i would say go to Las Terrenas... (snorkling, more beaches, etc.) & Colibri is nice ... there are others as well. Las Galeras is a somewhat short drive to the end of the penninsula and you can always go there for a day trip or an overnite...especially for the whales. About a 45-50 minute trip from LT.
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Wait and see who else responds as they may have other opinions/options...good luck...sh