The pub's storeroom is at the right of the kitchen door, to the right of the vocabulary game.
The general rule is to take everything you can, whether you know why or not. In this case, that's a canister of gasoline and a disk with a flower on it. Just hold onto them; you'll know when to use them.
And while you're in here, you'll overhear an argument between Gunnar and Elisabet. What an odd man he is!