Meld into Stone seems the obvious choice to me (maybe with a shorter duration).
While an obvious choice, it also seems to be, from what we have thus far, an overpowered one.
Meld w/Stone is a 3rd level spell/effect.
Air Gen's get Levitate (2nd level) not Fly (3rd).
Fire and Water get a cantrip and 1st level spell at 3rd level.
Extrapolating, other PC races innate magics are basically cantrips and 1st level spell effects with Drow magic topping out on a 2nd level spell (darkness).
Yes, Svirf's have at will nondetection. But that really just goes to their history/legacy of innate powers for the history of the game (and is pretty much the only explanation how this race of helpful "good guys" can survive in an Underdark full of magical and psychic foes). They are the obvious proverbial "exception to the rule" (and have always had/been as imbalanced in terms of innate magic as all of the other Underdark races).
It seems the determination was made somewhere in the design process that a 3rd level spell effect isn't really in line with the "power balance" for innate magics that 5e has been presenting.
So, while I agree, it is the obvious flavor kind of choice. You can get/have just as much earthy flavor with appropriate level spells (and maybe some minor refluffing) with something less than 3rd level.