one way is to write new words to a song. Then write a new tune to the words. If you chose a dance tune that has words you can ditch the new words and what you have is a new tune that has the same format. Worth a try is you are in a hurry, new to it, need a lot of tunes or generally jaded with other aproaches. The work may be derivative but sometime you will have worth that you can only dream of. I calls it Tested Words In Substitute Tune - TWIST
Ian Bruce reckoned he picked-up the wrong guitar one day and launched into a tune. The chords were different but as he was just musing he investigated the possiblities immediately. He is right handed I think and the guitar had been for a left hander in mirror inmage. Find your left handed guitar
We are coming near to autumn here in the UK so birds flock ready to migrate - look at the pattern of the birds on the wires if you can find the right telegraph poles.
Try playing tunes backwards.
and try converting waltz to polka (OK so it ain't that easy but some folks can do it on the fly).
Apart from Twist these may only give you riffs, chords, runs etc but it's a start.

Mutton Chops & I once tried to set the Words of Grays Elegy in a Country Churchyard to music. We did not know them as such and found them in a churchyard, which he did & I said yesno to. One note was causing difficulty then: As the Church Bell Chimed
Eureka! And that is what we called the song.
Inspiration comes from everywhere - the skill is in plucking the right fruit.