As of version 0.36, Treasure Chests were added to Adventure High. They can be found in the dungeon.
There's currently two.
For dialogue related to the Treasure Chests, refer to the Treasure Chest Transcript.
Sarah's Clothes
This one requires you to have Doll Sarah in your party. It's located on floor eleven.
In other words, you must have turned Sarah into a doll, then used Boost to change her back to normal.
From the entrance to floor twelve, head right, down, left and straight up. It's in the right most "room".
It contains Sarah's clothes, though they're more pink and quite revealing.
Back in the Protagonist's room, the Protagonist can give them to Sarah to wear. She agrees to do so, though she seems hesitant in doing so.
Once she wears them, she calls herself a "Doll" and says her name is "Funtime Sarah".
There is currently no permanent way of making her wear it.
Milking Pumps
This treasure chest requires Willing Slave Cassandra in the party or to be on the Submissive Route as Rosa.
This treasure is on floor thirteen, in the top left corner. It's a pair of magical breast pumps.
Using them on Cassandra four times while Sarah is in her Tough form causes an alternate scene to play out, in which Cassandra simply moos instead of speaking, annoying Sarah heavily.
This also leads to the start of the Cowsandra Conspiracy route.
If you're on the Submissive Route, Cassandra will use the milk pumps on Rosa back in the room.
Cassandra will also remark that she has not one, but two cows, to milk. A scene plays out in which both Rosa and Sarah are milked.
After being milked by the milking pumps, Rosa will overpower Cassandra and start milking her using the pumps, being unable to control herself.