Fancy house at each boss spawn.

No death run.

Seeing how far you can get with each armor type.

Seeing how far you can get before you kill the next boss. <optimizing progression>

Finding optimum base placements for reduction of transport. <trying to play with ore restricted in portal AND not transporting by boat.>

Doing a playthrough without the trader. <no weight belt, no headlight etc>

These are some of the things I have done to keep me occupied.