I literally ran this for my group just now.

Background was a cave system with very limited resources (previously few dead bodies to spare) and the Lich was both rather low on the power scale and Artificer-themed (so no "just spellcasting the party to death) and a Kobold.

So he fought/thought more like an engineer, not a powerful necromancer.

Highlights included:

• Lair filled with Carbon Monoxide (I let the party notice symptoms and retreat, it was more of an area dental tool than a trap)

• freezing the flooded exit tunnel so the party had to tunnel through ice, making them vulnerable to an ambush

• installing an artifact that would flash-freeze said flooded tunnel with the party inside

• a "main entry" labyrinth to the lair that was rigged to collapse

• a side entry with a hidden, massive door that tried to crush the party and did manage to separate them

• one-time sigils that would create a zone of Web, Darkness and, again, flash-freeze the intruders (and the undead defenders delaying them)

• a remote body with his true body walled off in a tunnel behind the wall it can escape even if it loses

• a few, highly upgraded undead