Slam seems to be the only attack an earth elemental can do? Can a druid wild shaped as a huge earth elemental crush a foe by stepping, sitting, or jumping on it?
The typical earth elemental has no greater capacity to deal damage to a foe by jumping on, sitting on, or stepping on that foe than any other typical creature does. (In other words, none.) For example, a Huge or bigger dragon has rules for crushing a foe beneath it, but an earth elemental doesn't. An earth elemental slams its foes, and, unless the earth elemental (or the druid the earth elemental actually is) has some ability that permits it do more than that, those slams—and, of course, any combat maneuvers it attempts—are all that the earth elemental'll do.
Of course, only the GM stops a player from describing an attack as something more visceral or cinematic. An earth elemental's slam needn't be described as just a wallop from it's mighty fist and could instead totally be described as a bellyflop on some unfortunate soul; a firm, ugly, hard squat on a foe's head; or it just stamping really hard on some poor dude's foot.
None of this description changes anything or causes the attack to have any additional or special combat effect, though—it's just description—, and sometimes the GM will even mandate that a slam involve the creature's hands or arms or whatever. However, until the GM says someone's gone too far into flowery cinematic descriptions, an earth elemental's slam can be described as pretty much anything the GM and the rest of the table will tolerate. Refluffing stuff like that is part of the game.
Note: Natural Attacks says, "Creatures with natural attacks and attacks made with weapons can use both as part of a full attack action (although often a creature must forgo one natural attack for each weapon clutched in that limb, be it a claw, tentacle, or slam)." That parenthetical means that a GM may rule that an earth elemental's slam is mechanically unavailable if the earth elemental, for example, also attacks with a greatsword, this despite the earth elemental wanting to describe the slam as heel stamp, body check, or flying leap.