King's Quest II: Romancing the Throne
Sometimes when arriving at Hagatha's cave, she comes driving out with a Bat Mobil.
Read the note behind the right-most tree in the screen north-east of where Neptune's trident is found. The note advertises Space Quest 1 and King's Quest 3.
Look in the hole in the rock at the cave entrance in the high land to see the trailer of Space Quest 1