Well, I don't mean epic, multi-map campaigns, just something small and quick, which will allow you to show off your entity-scripting skills in addition to design/gameplay skills.
Everts idea of a boss fight/battle map would be the best most likely. Something similar to the Strider battle in HL2, or the Gunship battle in HL2:Ep1 where you fought the gunship ontop of the hospital.