During the rainy season, expect road conditions to deteriorate severely. Major highways are reasonably well maintained all year (some better than others), but often the government will not bother patching other roads during the rains. As a result, many dirt roads become totally impassable, even with four-wheel-drive.
Also during the rains, water visibility for scuba diving is diminished on the Bay Islands [1]. But because the islands don’t have the high mountains like the mainland to catch precipitation, rainfall tends to be considerably lower than on the north coast. February–March and July–September are usually two good times of year for the islands.
Hiking in the mountains of central Honduras [2] is possible year-round, but it’s less muddy during the dry season, especially in the south part of the country. In places like Pico Bonito [3], Cusuco [4], or Sierra de Agalta, it rains all year, so just come prepared to get wet. If you’re planning on rafting or river-boating, the best time of year is after the rains, in December or January, when water levels are high.
Links:
[1] http://www.moon.com/destinations/honduras/the-bay-islands
[2] http://www.moon.com/destinations/honduras
[3] http://www.moon.com/destinations/honduras/the-north-coast/la-ceiba/south-and-west-la-ceiba/parque-nacional-pico-bonito
[4] http://www.moon.com/destinations/honduras/central-honduras/san-pedro-sula/parque-nacional-cusuco