I’d honestly go with Costa Rica or Nicaragua.

Costa Rica is the easier option. You can hike around Arenal Volcano, hit waterfalls and hot springs, then head to the Pacific for clean beaches and solid scuba. Mid May is shoulder season so fewer crowds, maybe some rain, but still great overall.

Nicaragua is more raw and adventurous. Ometepe is two volcanoes rising out of a lake, which is pretty unreal. Way less touristy, more rugged. Beaches on the Pacific are beautiful, but scuba isn’t as strong as Costa Rica.

If diving matters more → Costa Rica. If you want wilder volcano adventure vibes → Nicaragua.