25 Gallon (30x12x16)

What kinds of snakes could live their entire lives happily inside a 30"l x 12"w x 16"h terrarium?

Thayeri or variable kingsnakes stay pretty small, particular males and could be housed in a tank that size. Just make sure the lid is locking.

Sand boa, garter snake, hognose, king snake, children's python, spotted python, and rosy boas are some fairly beginner/intermediate species that would do well in a terrarium of those dimensions

I personally would go with a king. Plenty of morphs to choose from with plenty of patterns/color combos, etc... I would also consider a hognose.