Hi BNewman -- have you tried the Myst games? They have a first person perspective like Riddle of the Sphinx and contain challenging puzzles. You can buy the first three games
as a collection, which is a very good deal. You can also download them from places like Gog.com and Steam.
Another idea -- check out the games by
Kheops Studios. They have history-based games, including games set in ancient Egypt like RotS.