Make a scrap pile of wordstuff — nice words, clauses, phrases, fragments. Your pile should have maybe 20 things in it. You can make this scrap pile from your own environment by listing visible, audible, smellable or touchable things in your environment, or you might grab a few books and drop your finger down at random to cull wordstuff from their pages.
Then select some stuffs from your pile and form them into sentences, first by putting the stuff in an order, and then by writing whatever you need to do to connect stuff one to stuff two to stuff three and so on.
As an added filter, try imagining these sentences you are composing take different simple shapes: triangular, circular, square.
You’ll have to make some choices about what properties of circularity or triangularity are transferrable to sentence syntax. An even rate of change (circularity)? Three moments of sudden angularity or direction change (triangularity)? What else might a shape lend to a sentence’s possibility?
For maximum syntax warmup gymnastics, try using the same stuffs in different shapes and orders. Use this as a warmup for your language brain to remind yourself of how elastic the field of choice is in syntax: logic, order, and connectivity.