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."