I use actions regularly. Sometimes I just need to resize a bunch of images to a certain size, and will never use that action again. So I create an action, use it and then delete it, or let it rot in the actions dialogue.
Is there a way to make some changes to an image, and then apply to all open images without having to explicitly create an action?
As far as I'm aware there are only 3 ways to "repeat" something within Photoshop, and only 2 of those ways apply to using different documents.
Edit > Transform Again - Only works within one document and only
for some operationsTo this end, No. I don't believe you can repeat a series of steps across multiple (open) images without the use of actions or scripting. And as for set up, if the steps are always changing, actions are far easier to use than scripting since actions are edited within the Photoshop UI itself.
Perhaps I am looking at your question differently than others.
If all the images are in the same folder you could always create the action and batch process the folder with that action as tosave yourself from manually applying it to each image.
Really, actions could not be any easier being that you name it and hit record to save a process you would have to do anyways. Batch processing just takes the redundancy out applying it.
You can literally create an action, batch process, and delete it in only a few small steps (not including the action processes itself).
