Some I can think of off the top of my head:
Once in a blue moon
Stick in the mud
Colder that a witch's tit
Fixing' to
Darn tootin'
Knee high to a grasshopper
Skedaddle
Ornery
Piddlin'
My MIL says britches (pants), mash instead of press, as in "Go mash the button", taters instead of potatoes. My grandfather used to say 'reckon' and 'yonder' all the time also.