Wizards’ spellbooks and artificer "magic" are explicitly called out as being reflavorable–that is, as long as everything has the same numbers, a "spellbook" doesn’t need to be a book, and artificer "spells" don’t need to look like classic magic spells. I think I remember seeing some passage about being allowed to reflavor spells in general, such as learning magic missile but casting it by creating a floating skull that spews spooky blobs of pain (as long as the skull doesn’t do anything other than deal 1d4+1 force damage three times), but I skimmed the magic section of the PHB and couldn’t find it.
Spellcasting focuses and musical instruments are also given as open lists, including some example items and their prices/weights, but stating that, for example, an arcane focus can be "some similar item". What exactly this means for a character’s gold pouch and carrying capacity is unclear, but it is another example of something that explicitly allows players to–as long as they don’t receive any gameplay benefit–reflavor things to their tastes.
Are there any other rules like this, be they specific to a certain racial trait or class feature, or a more general "hey you’re allowed to change things up as long as they don’t affect gameplay"? I know that the DMG has guidelines for all sorts of alterations that you can make, but that seems like more of a soft suggestion than something that’s definitively legal. This question isn’t meant to be AL-specific, but using it as an example since it is an environment where DMs generally can’t just go "oh I like that idea sure you can do it"… Could an AL player have a Medium-sized non-venomous drider who’s bad at climbing by playing a drow and simply saying that their character is a drider*? Or could an AL player make a barbarian fight not by entering a primal rage, but by consuming a can of vitamin-rich spinach? Or, things in that vein that are related to some specific other race/racial trait or class/class feature?
*Actually, I just checked the AL rules and they say there that you can reskin races, so maybe AL isn’t the best example situation here. In general, I just mean "some setting where houserules and homebrew aren’t allowed."