Small flavour-based tweak, link each domain to unique way to get piety. Examples;

Good/Evil domains lose half the normal piety on each RPR Tick if the character is Good/Evil.

Animal domain gains piety on a successful Animal Empathy check.

Travel domain gains piety on every area transition.

Sun domain gains piety on use of Turn Undead.

Air/Fire/Earth/Water domains gain piety when summoning appropriate elementals or taking electricity/fire/sonic/acid damage.