For advanced exploration the Twilight Forest is hard to beat. Boss fights are available with unique rewards.
A challenging Sky Block
Ores can be processed automatically using an assembly line that creates material, pulverizes it then melts it down into a pure form.
Extra ores are available in The Nether to add some incentive for exploration.
Machines can be monitored via a screen. The 6 sides can be set to input or output independently.