The Mushrooms (and maybe the vases)

[JacksonTriggs Fan]

The mushrooms in the caves and secret areas are full of pills. I seem to have about a 50% chance of any given mushroom having something inside it when bombed, so with Azel flying I can get 4-5 mushrooms at once and get 2 pills and a coin.

Just something to note for others. Note that some mushrooms explode when exploded, with bad gas. So keep back.

[Siñor Stripper~Pole]

Ive also found that some of them (not sure if rocks are included) will have a trap door underneath it. This does not lead to the next floor. from what Ive seen ti gives a random /item/ and is just a bit of a bonus. :D

[PeanutBuddha]

Mushrooms also have a chance of dropping mushroom items, including Liberty Cap, Mini-mush, and the Magic Mushroom.

[Microsoft Excel 365]

the skulls in the depths seem to work the same way as well

[JacksonTriggs Fan]

The trapdoor is not always an item. One of them I went through led to a super devil shop with 6 offered items and a number of generic pickups scattered about. Unfortunately finding these trapdoors isn't as obvious as a vase or a blue rock with an "x" on it.

[JacksonTriggs Fan]

The pickaxe is now infinitely usable, so I'm really abusing that to check out the rates. The vases in the basement/cellar only contain coins and spiders, or at least only contain those at any realistic rate, having gone through about 50 of them without seeing other things. So unless you're running the pickaxe or Stomp, not worth breaking them.

It seems to be about 40% coins, 30% spiders, 30% nothing. With 1-2 coins appearing (2-4 with Humble Bundle powerup, I love that powerup).