The Beehive can be accessed by entering the portal contained by the chest which spawns after defeating the Hive or Swarm minibosses. This chest only spawns if The Player has been hit once or hasn't been hit at all while completing the miniboss room. Being hit two or more times will result in no chest spawning.
Beehive always has the same overall room layout: a central room with the portal which leads back to the main level, and eight other rooms, two in each cardinal direction. All four adjacent rooms to the central starting one have bee-type enemies, the rest contain a herb, the Honey item, or a dotted chest containing a random item. Herbs always appear in opposite directions, the same is true for items (e.g. if the left path leads to the Honey, right will have the chest, and the upper and lower paths, herbs). The herbs are usually, but not always, green herbs.
Since Beehive is a very small side level and doesn't involve too much risk for a more than decent reward, it is generally recommended to visit and complete whenever possible; Honey should be the player's main objective due to its usefulness (and the fact that it always spawns).