Will that chest give me a beautiful gem or a book filled with dark magic and curses? When designing treasures and items you need to balance usefulness, fun, and mystery. They should have hope when seeing the card first flip over then a little touch of fear should creep in as they remember past times when they found treasures and barely survived.
First, focus on something useful in gameplay but you do not want to give something all-powerful- that takes the fun away. Some items players need a lot of, like arrows and shields. Others like speed and extra reward you do not need a lot of. Here are some example items, see one card ahead one use only, gain ally 1D20 turns, extra parts, + 1D6 speed, 1D4 villains flee, double reward next two attacks, 10 health use one time, +4 attack, torpedo,1D6 teleport.
Next, focus on the fun! Weave a story into the treasure or make the reward unexpectedly strange, Here are some example treasures, a mysterious trail of tiny footprints that has gold 1D4 gold for 1D6 turns, or a glowing gem cause 1 villain to flee. A treasure can also be a living thing or Non-Player Character (NPC), like a plant that grows gems, a talking book, or a pet. Usually the NPC does not last for the entire game but a set amount of turns. Remember to try and incorporate miniatures into the gameplay. For example, when you have a pet you put a figure next to your character.
But players should be on their guard when finding treasure! Some treasures can be cursed it could do worse. A player might find they have a powerful magic book but, it is cursed unleashing terrible dark magic when used. Or the player found a +4 sword that has a terrible lich guarding it. Other ill effects can be harder to detect, like a cursed item that causes all initiative roles to fail or a pet who attacks when entering cities. Another example is the reward is a trick, like leprechaun’s gold which dissolves into dust after a few turns.
If you are playing in space then the items or treasures can not be magical, on land they can not be things like torpedos. You can get items by going to a space station/civilization or by encountering a merchant.
Treasures can be used to just advanced the game plot like other cards. Also, as with all cards within HarmoniousWorlds, try and make the cards play out differently each time. Every time a player gets a card the events that unfold and rewards should be different. Take to make sure skill, as well as luck, determines the outcome.
May the dice be in your favor!