The coin fits into the design carved on the back of the hippo like a key. This looks like the slot. Now let's see what it unlocks.
9 seconds sound clip from Scooby-Doo, Where Are You! - Scooby Doo and a Mummy, Too (S1E12).
You can hear this line at 00:20:09.382 in the Blu-ray version of the episode.
Quote context
[...]
- Well, Scooby Dooby Doo. Whoops!
- Now let's see who's really behind these bandages.
- And when he couldn't find the coin, he knew one of us must have it.
- So he devised this whole mummy plan to scare us into giving it to him.
- But we wouldn't give it to him, and later we figured out the secret.
- Wow! A secret compartment with a glass beetle in it!
- Not quite a glass beetle. It's Anka's most valued treasure... a diamond scarab worth a fortune.
- So that's what he was after!
- But there's still one thing unsolved. What happened to the real mummy?
[...]