For example, a treasure track artifact that you can create or maybe find in hidden chests. That artifact will beep if you are close (500 blocks) of a treasure. It will increase the bipping intensity when you get closer than 200 blocks, 100, 50, 20.
That won't give you the exact location of the treasure, but a close one. Then, you'll have to dig and find the treasure, that probably it will be randomly in a cave full of monsters, in depth 0-30.
That treasure can be a decorative object (random from a set of maybe 20 different). Is no use except for showoff (maybe a random sculpture). It's a propietary item, so if it's destroyed by anyone in SMP, the item goes back to the inventory of the owner.
Then, a player can decide to go to search treauses, because he's boring. He wanders around, discovering new places, until it get the beeping. Then, he get as close as he can in the surface (less than 50 blocks of distance, so a intense beep), he make a camp, and start digging or looking for open caves in order to search that item.
After a few days of spelunking, he get the treasure. He got lucky, is not a repeated one, is one of the 8 unique treasures that he has left. Then he go back to his home and add the new treasure in the trophy room, and think about going south instead of north to see if he find another treasure, or maybe he wants to wander north again, because in the path of the last treasure he found a good place to make another castle.