My favorite one: “Consarn it!” - Granpappy Amos McCoy (Walter Brennan)
Here it is hidden in a phrase along with another subsitute word:
“GOT down offa my horse, and stepped in a pile of shifting sand!” - anon.
I know y’all must have some faves…
Q
Goldurn Euphemisms, surely? Goddamn is a little too explicit for this trope.
My favourites are from those American 'tween-wars novels, where such fossilised expressions as Lawks a-Mercy! and Lan-sakes! occurred.
Well, I was trying to be clever. I guess I could have written “Euphemisms for ‘Godamn’”.
“Goldurn” is good though! 
Quasi
My dad used to say, ‘Cheese and crackers got all muddy!’
Well, spit! I don’t have another farking thing to add, not after I fudged this up as badly as I have.
Situation normal, all fouled up! Doggone it!
W. C. Fields was famous for pushing the envelope of what he could say in his films. My favorite was “Godfrey Daniel!”
Dad gummit
Dadburn rick-em rack-em!
(Courtesy of the Yosemite Sam School of Creative Cursing)
And if that doesn’t qualify, y’all can just go punk yourselves. 
Dag nabbit! (Another Walter Brennan favourite).
Fart-knockers.
Instead of Jesus Chirst, people here say “Cheese and Rice.”
I’ve always wondered what Jesus Christ Superstar lyricist Tim RICE thinks of that.
Bollards!
Shut the fffffffront door!
Gosh dang it you son of a bird, this is a tough one! 
I’ve always been partial to “Ye gads!” and “Gadzooks!” That said, I rarely have the restraint to use them…
That reminds me of another dad-ism: ‘Ye gods, and little fishes!’
My friends Mom while she was with my friend and granddaughter:
Oh fuckrying out loud!
(Nice save Mrs. D)
Miss Susie had a steamboat, the steamboat had a bell,
Miss Susie went to heaven, The steamboat went to…
Hello operator, please give me number nine,
And if you disconnect me, I’ll chop of your…
Behind the fridgerator, there was a piece of glass,
Miss Susie sat on it, and broke her little…
Ask me no more questions, tell me no more lies,
The boys are in the bathroom, zipping up their…
Flies are in the meadow, bees are in the park,
Miss Susie and her boyfriend, are kissing in the…
D-A-R-K, D-A-R-K, dark, dark,
Dark is like a movie, A movie’s like a show,
A show is like a TV set, and that is all…
I know I know my ma, I know I know my pa,
I know I know my sister, with the 80 meter bra!
My mom was born in England, My dad was born in France,
I was born in diapers, I couldn’t fit in pants.
My mom is Godzilla, My dad is King Kong,
My brother is the stupid one, who made up this dumb song!
Gorram fracking sci-fi euphemisms!
movingfinger - I saw a televised version of a Tarantino movie (Jackie Brown, I think it was) in which practically every line out of Samuel L. Jackson’s mouth included a variation on “monkey licker,” “melon sucker,” and so on.