[3.5] Easy-Entry Divine PrCs

[Telonius]

Thaumaturgist; one feat (that you probably want if you want to be summoning things anyway), and the appropriate casting level, and you're in.

Radiant Servant of Pelor (assuming you worship Pelor). Extra Turning is something Clerics tend to get (whether or not they take the PrC) just to fuel Divine Metamagic. Add a couple of skills (Know Religion and Heal) that most Clerics will put ranks in anyway.

Knight of the Raven. The prereqs consist of being Good, staying up all night once, and having enough BAB. (Ravenloft-specific, though). Not quite full casting (9/10), but it gives an awful lot of goodies. (One of the rare full-BAB PrCs that a Cleric might take, plus access to the Sun domain).

Bone Knight. Ride is cross-class, but you can qualify starting at level 9. Otherwise, Craft and Knowledge (Religion), plus appropriate BAB. Again, only 9/10 casting, but it's close to full and gives a ton of interesting abilities for the Cleric who's looking to build an undead army.