Duck around the stairs for now, and shove the Southern of the two floating rocks to the West out of your way, so you can slip by between them.  On the edge of the drop to the lower floor you’ll find a Magnet, a hold item that boosts Electric attacks by 20%.  Not a bad item choice overall, though there are more universally useful hold items you’ll find later on.

Return to and head down the stairs from here.  Off to the left you’ll come across a father-son pair waiting by a staircase up.  These are the Nugget Man and Nugget Boy, who will give you a Nugget and a Big Nugget, both of them high-value items you can sell for cash.  Once you’ve recieved your reward for finding them, head off to the West.  The pathway turns North fairly quickly, under a set of three blue rocks that float very high up, then ascends some stairs to the West.  Climb these and you’ll come out in a small otherwise-unreachable area on the main floor of the cave.

Here you’ll find a girl who gives you the ‘Find Mysterious Ores’ Funfest Mission, because she wants pretty rocks and jewels to... stick to the walls of her room?  Curious sense of decor, she has.  You can also find a Pokemon Ranger here.

Battle: PKMN Ranger Louis (2040 Poke, chesto berry)
Opponent: Ampharos Lv 34

Louis’ Ampharos loves to hit with both paralysis via Static and Thunderpunch and confusion from Confuse Ray, a dangerous combination that often leaves your pokemon alternating between flinching and beating itself up.  Even ground-types will be exposed to its Confuse Ray, so your safest option is to hit it very hard right off the bat in order to avoid the aggravation.

In addition to the Chesto Berry Louis gives you, you can step south and look under the glowing rock for a bottle of Carbos.

Go back down the stairs to the Nugget father-and-son combo and you can climb the small stairway up to the higher layer of this floor.  Head East immediately and bypass the various glowing rocks.  The walkway narrows a good bit, but keep on it and you’ll run across a Pokemon Ranger.

Battle: PKMN Ranger Briana (1980 Poke, chesto berry)
Opponent: Minccino Lv 33, Excadrill Lv 33