you saw the title. were making a homebrew dungeon inside a giant frog. share your ideas below

Colony of giant flies.

A nearby colony of bullywugs or grungs consider it a god and throw sacrifices to it, like Lord Jabu-Jabu but frog. There is thus a trove of valuables somewhere inside.

It secretes a psychedelic toxin. Roll a Constitution save or be charmed.

A Mind Flayer it ate has been living off its brain in turn for years.

Oozes. Just... lots of oozes.

A cursed +3 sword whose sole drawback is making anyone who attunes to it permanently and irresistibly appetizing to frogs, and the body of its unfortunate wielder who assumed it was worth the risk.

A bump on a log in a hole in the bottom of the sea.

First, I need more information. Is the giant frog?

• Dead? : furniture carved out of frog parts.
• Undead?: elaborate control machines to manipulate the frog.
• Alive and being tortured: fiends that are living in it.
• Alive and unaware anything is inside it: mimics, purple wormlings (large sized)

Hm. Canon about Ramenos is rather thin, but looks like you can expect bullywugs, slaad, frogs, and the like.

A thin sheet of metal with numbers and letters pressed into it.  The front side is painted to make the numbers stand out and a little tag on it indicates it expires ‘Feb. ‘88’.

A Purple Worm permanently reduced, to small size.

So from folk stories: 

• There's a story about a frog that swallowed all the worlds water so why not an entire river along with it's water spirit. It could be a fey, dragon or elemental. 
• Otherwise there's also the naruto thing where you have a giant frog v.s a giant snake with a giant snake inside the frog. 

you could combine the two and have like an entire forest with a river and a water spirit that asks you to slay some giant demonic serpent inside a giant frog.

• Hags, witches turn things into frogs so why not a hags lair? 

From animals : 

• There was a species frog that gestated it's young in it's stomach. So why not acid proof frogs in a stomach acid swamp. 

Mandlebrot the Frog God.....inside the Giant frog is another giant frog, and inside that..another giant frog...its effectively a demiplane/maze/imprisonment spell, no matter how many times you are swallowed or otherwise enter a frogs mouth you experioence momentary darkness before emerging in a swampy area where another giant frog sits, waiting to swallow you.

Lots of random trinkets, the remnants of stuff the frog had previously eaten.