If you head to the left from the entrance, you can make your way down a stair to the basement.  Here you’ll find a second Spell Tag (just in case you needed it), though you can’t reach the other Item Ball you see.

Bracketing the stairs leading up from the first floor are a pair of doors- the right hand one leads to a room with a backpacker in it.

Battle: Backpacker Boone (864 Poke)
Opponent: Numel Lv 36, Sigilyph Lv 36

Head further right to the stairs up and cross the balcony area (making note of the ghost that briefly appears), coming down on the left to enter the room on the left side of the central stairs- here you’ll find a Full Heal.  Duck out and into the basement- you can get to the other item down here now, which is a Dusk Stone, the evolution item for Murkrow, Misdreavus, and Lampent.  You’ll then want to head right across the basement and up the other stairs a second time.

Ascend to the balcony level again, and enter the right-hand room on the top floor to claim a Rare Candy.  Then return to the ground floor and head off to the left.  This will let you get to the room atop the left stairs, which holds another battle for you.

Battle: Psychic Lynette (1152 Poke)
Opponent: Drifloon Lv 36, Drifblim Lv 36, Spoink Lv 36

After defeating the psychic, return to the main floor and then climb the central stair to the last room on the top floor of the building.  You’ll spot the ghost on the balcony again before entering, and in the center of the room is the item the ghost mentioned- the Lunar Wing.  This item is particularly interesting because it allows you to reach and capture Cresselia far later on during the game, when you cross the Marvelous Bridge.

On picking up the Lunar Wing, the ghost will reappear and ask you to bring the item to the pokemon to which it belongs- exactly what you’re going to do later on.