Arenal Observatory Lodge is the closest hotel to Volcano Arenal. It was built by the Smithsonian Institution, which wanted to study the volcano, and the older rooms have that "decorated by a geophysicist" look. Fortunately, they were just finishing some new rooms when we visited (January 1995) and the restaurant/lounge is very pleasant.
Most people visit the lodge on a multi-day tour possible including
Monteverde or Cano Negro, but room rates by themselves are about
$70/night. Book through Costa Rica Sun Tours, Apdo. Postal
1195-1250, Escazu, Costa Rica, voice (506) 255-3418, FAX (506)
255-4410. US mail drop: PO Box 025216-1660, Miami, FL 33102-5216.