Idea: Diamond Block Retrieval Course

Find a lava lake in the main world. Carefully place sand in several different spots scattered on the lava lake, and put cacti on them. (Cactus damage may be off but they'll destroy item entities). Carefully place the diamond blocks above the lava lake and cacti. If you have silverfish cobble/stone bricks, put some among the diamond blocks. Enclose the entire lava lake such that there is only one entrance. Rig up a locked dispenser filled with splash potions of slowness, and a pressure plate, such that those who enter will be slowed. Let a person enter the area and collect as many diamond blocks as they can. The only rule is to not use water buckets.

Optional: Make it a parkour course
Optional (2): Add dispensers filled with splash potions of poison scattered throughout the course, or look for a lava lake near a cave spider spawner
Optional (3): Make a mob farm system that lets mobs enter the course as the player tries to finish it.

Give them to me and I throw them into a cactus.