Why didn't Krillin and Gohan store the Dragon balls in a capsule?
I haven't come across a solid answer yet, but http://dragonball.wikia.com/wiki/Capsule might have an explanation.

In the article, they mention that a capsule can't be stored within another capsule (as "the results were catastrophic") so in that regard it is possible for an object to impact a capsule, at least negatively. I then wonder if storing a Dragon ball would negatively impact it, as Colonel Silver in Dragonball said that the balls were unbreakable (and by unbreakable, perhaps unmodifiable and therefore unable to be stored)

It also mentions that "organic corpses" were stored in capsules, but no sapient creatures. To me, this sounds like no "animal testing" has occurred (possibly because a lot of the universe's inhabitants are talking animals and it would be morally / ethically wrong) but I don't know.

So this may be a partial answer, but it'd be nice to have "official" confirmation of why these things haven't taken place before.