Not every chest or puzzle needs to be a simple monetary reward, like a pile of gold. There is magic in D&D so the results can be more. . . abstract. Here are d4 things that can be locked away.
- Portal. By solving this puzzle or unlocking this device you open a portal to a keyed plane. The portal remains open for as long as the puzzle remains solved and the key unlocked.
- Memories. Sometimes the best thing someone can do is forget what they’ve done. These locked away memories allow the person to live without the guilt or weight of their crimes. Anyone that solves the puzzle or unlocks the device is immediately flooded with the memories and knows what was done.
- True Nature. The locked object is just a polymorphed creature that has been imprisoned. Once the object is unlocked the object transforms back into the original creature.
- Lock of Ages. This object can be unlocked or solved in two ways. You can magically see whatever happened 100 years ago within 60 feet. If you solve it the other way you can magically see whatever happened 100 years in the future within 60 feet.