Astrology could be lots of fun...
"So, Krusk, you were born under the sign of the Emu in the year of the swine..."
"Today you will meet a great challenge and have a relationship crisis of some sort"
"The moon is in the house of Hippie. Better not kill and eat that cow, doc..."
Astrology might also know some obscure mythos, beasts of olde, the planets, etc.
Astronomy would probably be a wizard at timekeeping (say, a DC 10 check to figure out the exact hour it is). He'd know about orbits, swinging, etc.
If you want, you could give competing theories and have him make checks based on which theory is true. Is it clockwork? Are they lines or circles? Is it a vacuum or some quintessence?
He'd probably have some rudimenatry planar lore...be able to intuit direction fairly easily...maybe know exactly what the sun is made of and how the moon can change form like that.
Think of a variant lycanthrope who only changes when Mars is ascendant. The players have been pestered for a week during a full moon, and think they have a few more to try and uncover the secret...but it keeps happening.
I love the star-based PrC idea. Use it.
Mathematical puzzles based on the formulas for orbits or the cycle of the stars through the heavans is good.
Yeah, that'll do.