We have a silver treasure chest (seen by chance on sale at Dunelm Mill!) but this would work equally well with a box covered with shiny paper.
Into the box we put ordinary household items or foods such as cups, soap, crayons, raisins (if we are telling a story we try to link the items to the story e.g. for Moses crossing the Red sea, we might put in a bottle of water, a shoe, a piece of stick.) When it comes to a time for prayer, we let the children take the items one by one out of the box, discuss what they are and why we need them (and how they link to the story) and then together we say 'Thank you God for...' They especially love being chosen to open the chest and choose an item!