Many items and artifacts from throughout the series make an appearance in BMO's cave, including:
The Prism-gram from "Her Parents"
A painting of an elderly and dignified Banana Man
Kim Kil Whan's King of Ooo statue from "Ocarina"
Margaret's music box from "Jake vs. Me-Mow" (now broken)
Princess Bubblegum's Bod-Rod from "Bonnie and Neddy" (now broken)
The Rainicornicopia from "Paper Pete"
Lemongrab's sound sword
Lumpy Space Princess' star "jewel"
Jake's golden tooth from "Gut Grinder"
Flame Princess' lamp
James Baxter's beach balls
The Door Lord's door hat from "What Was Missing"
Jake's hat from "Dungeon Train"
Ice King's drums
Part of one of Betty's jumper cables
Finn and Jake's treehouse clock (now broken)
Grob Gob Glob Grod's helmet
Lady's universal translator from "My Two Favorite People"
The "Cool Guy" mug from "Card Wars"
Dirt Beer Guy's jar from "Cherry Cream Soda"
A magic wand from "The Pods"
One of the speech cards from "Shh!"
Jake's favorite mug (smashed and glued)
The bricks from Jake's cabin in "Jake The Brick"
The Book of Royal Rules from "The Silent King"
A copy of Jay T. Doggzone's "Mind Games"
A block from the tower Finn builds in "The Tower"
The Fionna and Cake Omnibus (deceased)
An axe from "Shh!"
Ice King's hat from "The Party's Over, Isla de Señorita"
The Jiggler's casket
One of Magic Man's "dirt" boxes
The butt chest from "Vault of Bones"
A can of Softy Cheese from "Ghost Princess"
The Snow Golem's wreath from "Thank You"
Princess Bubblegum's war horn
A box of Super Porp from "Dark Purple"
Part of Lemonhope's crafted harp
A shard from Bronwyn's skateboard from "Wheels"
One of the wood cutouts from "Orgalorg"
Stakes from the Stakes miniseries
The basketball from "Simon & Marcy"
Engagement Ring Princess' ring
Marceline's sunscreen lotion
Colonel Candy Corn's medal
The Marauder leader's helmet