is the Box of Untold Secrets something you can open??

Nope sadly. It can be used as a toy however.

Guess we will never know all it's secrets :(

haha ok.. what kind of item can be opened agian... i forgot

There are many sorts of items that can be opened. If you have an item in your inventory, click on it and if it can be opened you will see an "open" under Other Options. Things like parcels, bags, boxes, collections, are all more likely to be openable.

You can check individual items at SubetaLodge, or do a notes search like this Notes Search.

Some items that can be opened might have the "use" option instead of "Open" like the trading card Booster Packs.