What ways are there for you to have a character who can shift, fairly quickly, between a small number of forms with completely different stats?
Polymorph chain, wildshape (most powerful option, but
too many possible forms)
Body of war and similar spells (single-form polymorph with longer duration)
Lycanthropes and other creatures with Alternate Form
Momentary Alteration feat (short-duration
alter self, one form only)
Chameleon (not so fast to change, but worth mentioning)
Incarnate shifting around essentia
Binder with the Expel Vestige feat
Tenser's tranformation,
mental pinnacle, etc.
Barbarian gish (especially bear warriors, goliaths with Mountain Rage, etc.)