Play-on words are as fun to read as they are challenging to write: https://sarahleastories.com/2015/04/02/writers-digest-poem-a-day-challenge/
Pick a noun and write every word you can think of associated with it. This exercise will sharpen your ability to play with play-on words. https://sarahleastories.com/2018/05/09/writers-digest-wednesday-poetry-prompt-435-reserved/
I took a list of root operations (i.e., resection, extirpation, fragmentation, etc.) I learned from my medical coding classes and creating a series of poems that implement one of those processes and newspaper jargon. The results are fun and surprising. https://sarahleastories.com/2018/07/23/something-for-everyone-resecting-at-the-sunday-breakfast-table/
When I’m stuck, I come up with a list of opposites and how I can link them. https://sarahleastories.com/2017/02/07/micropoetry-monday-opposites/
Writing about our crazy language can be fun. Pick some words that vex you and explain why they do. For me, extraordinary would seem to mean its opposite—extra ordinary.
Know your craft, but play around with words. Have fun with language. (Shakespeare and Lewis Carroll did, among others.) You can still be literary without having to be literal. https://sarahleastories.com/2014/03/25/fun-with-words-lists-are-fun/
Noodling around with words germane to a certain subject be it math (https://sarahleastories.com/2017/11/10/writers-digest-november-poem-a-day-2017-challenge-10-theme-going-somewhere/) or English (https://sarahleastories.com/2017/11/13/writers-digest-november-poem-a-day-2017-challenge-13-theme-city/) can be a fun way to use old words in a new way or even learn new words, as every vocation or discipline has its vocabulary.
Dictionary.com has a Word of the Day, but sometimes those words, although fun to know, you will probably never use, as they are archaic (and would only come in handy if you are writing about that particular time and place). If you’re seeking more avant-garde terminology, try Urban Dictionary. Even if a word doesn’t have a place in your speaking, it might have a place in your writing. https://sarahleastories.com/2018/04/25/poem-a-day-april-2018-writers-digest-challenge-25-theme-intriguing-seldom-used-word/
Take a bunch of related words and see what you can cook up. https://sarahleastories.com/2018/04/18/poem-a-day-april-2018-writers-digest-challenge-18-theme-temptation/