I use an uncomfortable amount of these.
Speaking of “It’s A Wonderful Life,” it contains a euphemism for pregnant that I am using at every opportunity: “on the nest,” as in, “Are ya, Mare?! Are ya on the nest?!” I love it.
My personal favorite for a moment of surprise
“It just goes to show that wherever you go, there you are.”
It beats the alternative -
“Well, I’ll be dipped in shit.”
“Please” and “thank you”
I’ve personally been making an effort to say “great googly moogly” more.
In homage to one of Tuckerfan’s favorite crazed authors, I’d like to see people introducing debates by asking “Malarkey? Or effective way?”
“Howdy”
“Fellow”
“For Pete’s sake”
“Jim-dandy”
“Pyramid of human skulls”
I use howdy all the time though rarely have call for “pyramid of human skulls.”
Learned at my father’s knee: “Well, shithouse mouse!”
“My Stars!”
“That’s the cat’s pajamas”
and
“Knee-high to a grasshopper”
They all remind me of my late grandfather. Me and my sisters call them oldyisms. 
“No worries”
“No Drama”
I homesick for Australia!
We need more …
Toodle Pip,
scally-wag,
Hi Opal,
Hot-diggity,
Gordon Bennet,
and diddly-squat.
Cheers, Bippy
“Poppycock!”
“Horse patootie!”
“Quite right, old bean.”
“Tally-ho!”
My daughter recently mixed expressions, and came up with this gem:
“Oh, for crying out Pete!”
We love this and now use it all the time, much to her dismay.
“Laws, yes”
“I’ll be back anon”
“For the nonce”
“I’ve come over all (hungry, tired,etc.)”
I always heard this one “crazy as a shithouse mouse”.
Aparrently, shithouse mice are crazy, driven so by the demands of living in the shithouse.
Are you implying that my father didn’t create “shithouse mouse”?
Another childhood illusion down the drain . . .
He might have. Texas isn’t too far from Louisiana, it could have migrated. 
“There’s the rub.”
“Cool beans!”
“I’m fixin’ to go crazy, wanna come?”
“Ohhhh, that’s the Bees Knees!”
How about answering everything with a rhyming name (Popular in grade school):
“Sure thing, Bing.”
“No way, Faye.”
“Way to go, Joe.”
. . . ad infinitum.
“I never was one to speak my mind”
“I’m a bit sane, me”
“I just hate a challenge”