Ships, fighters, etc., stuff that might not even really factor in the end, but I find it helps during those days where I just can't be motivated to write. It helps me nail down some details I otherwise wouldn't think of - thinking out some of the details of vehicles and equipment helps nail down some of the minutia and physics of my universe, or what kind of motivations the people who designed them have (gives some insight into their motivations for other things).

I do the same as you, I have lots of ideas but when it comes to pen to paper, or for me fingers to keyboard, I find it hard to write those new ideas. So I tend to form new ideas and add to an existing world I already have, whether it's a new city, kingdom or race that I fit into there nicely.

I create races mainly, mainly the stereotypical lizardfolk, felinefolk and waterfolk to put them basically. I also use the same amount of characters I have done in my older stories, sometimes scrapping them altogether so that I can use the same characters in my new story and it keeps going on in circles.

It helps me brainstorm and relax and clarify the image of a character/race/area/item, especially if I'm having writer's block.

I have my MC running through corridors and around the periphery of a starship. My own mental picture wasn't enough. I drew out the diagram and have been back to it several times. Instead of getting in the way of writing, it cleared my view and made the descriptions consistent. The AI are often shown giving directions around the starship with that diagram. If the story ever gets to the point where somebody would be illustrating it, I have one graphic ready.